Electronic circuit, module, and system for radio wave powered battery switch

ABSTRACT

A convenient electronic circuit in which a switch is able to be switched through electric power obtained by weak radio waves is provided. An electronic circuit includes: a power supply configured to output direct current (DC) electric power; a switch connected between the power supply and a load driven by DC electric power supplied from the power supply and configured to switch a connection state between the power supply and the load from a non-conduction state to a conduction state; an power input terminal to which electric power obtained by radio waves received by an antenna capable of receiving the radio waves is input; a DC power output terminal configured to output DC electric power, a power conversion circuit configured to convert electric power input to the power input terminal into DC electric power and output the converted electric power from the DC power output terminal; an input terminal connected to the DC power output terminal of the power conversion circuit; an output terminal connected to the switch and configured to control a connection state of the switch; and a control circuit configured to control a connection state of the switch to be in a conduction state when the power conversion circuit outputs DC electric power due to the reception of radio waves by the antenna.

RELATED APPLICATIONS

This application claims priority to Japanese Patent Application Nos.2020-036246, filed on Mar. 3, 2020, and 2020-201341, filed on Dec. 3,2020, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an electronic circuit, a module, and asystem.

2. Description of the Related Art

In the related art, in electronic keys used for key systems forvehicles, electronic keys communicate with the vehicles when batteriesbuilt into the electronic keys are connected to control circuits usingthe electric power obtained by the radio waves from the vehicles.Techniques for reducing the consumption of batteries in a standby stateby cutting off the connection between batteries and circuits again ifelectronic keys are distant from vehicles (that is, during standby) areknown (for example, refer to Patent Document 1)

PATENT DOCUMENTS

[Patent Document 1] Japanese Unexamined Patent Application, FirstPublication No. 2011-24332

SUMMARY OF THE INVENTION

In the related art as described above, an analog front-end circuit (AFE)inside an electronic key is started up using electric power obtained byradio waves from a vehicle. At this time, the AFE is started up usingelectric power obtained by radio waves from a specific apparatusprovided in the vehicle or the like.

However, when the AFE is tried to be started up using radio waves fromthe vehicle, the electronic key has a problem that it takes time tostore electric power required for starting up the AFE. That is to say,it is inconvenient to have to keep the electronic key close to aposition in which the electronic key can receive radio waves from thevehicle until the electronic key is started up.

The present invention was made in view of such circumstances, and anobject of the present invention is to provide a convenient electroniccircuit in which a switch can be operated using electric power obtainedby weak radio waves.

An electronic circuit according to an aspect of the present inventionincludes: a switch which is connected between a power supply configuredto output direct current (DC) electric power and a load driven by DCelectric power supplied from the power supply and which switches aconnection state between the power supply and the load from anon-conduction state in which the supply of electric power from thepower supply to the load is cut off to a conduction state in whichelectric power is supplied from the power supply to the load; a thepower conversion circuit which includes a power input terminal to whichelectric power obtained by radio waves received by an antenna capable ofreceiving radio waves is input and a DC power output terminal configuredto output DC electric power and which converts electric power input tothe power input terminal into DC electric power and outputs theconverted DC electric power from the DC power output terminal; and acontrol circuit which includes an input terminal connected to the DCpower output terminal of the power conversion circuit and an outputterminal connected to the switch and configured to control a connectionstate of the switch and controls the connection state of the switch tobe in a conduction state when the power conversion circuit outputs DCelectric power due to the reception of the radio waves by the antenna.

Also, in an electronic circuit according to another aspect of thepresent invention, the control circuit may further include a powersupply terminal configured to receive electric power supplied from thepower supply, and when the switch is switched to be in a conductionstate, the control circuit may keep the switch in the conductive stateby electric power supplied from the power supply to the power supplyterminal.

In an electronic circuit according to another aspect of the presentinvention, the control circuit may include a flip-flop circuitconfigured to switch a control signal output from the output terminal.

In an electronic circuit according to another aspect of the presentinvention, the control circuit may include an electric power detectorwhich includes a reference input terminal, a detection input terminal,and a voltage detection output terminal configured to output a potentialaccording to a potential of the detection input terminal and a potentialof the reference input terminal, and the detection input terminal may beconnected to the voltage detection output terminal.

In an electronic circuit according to another aspect of the presentinvention, the electronic circuit may further includes a resistor inwhich a resistance value is 10 mega-ohms or more, wherein the detectioninput terminal may be connected to the voltage detection output terminalvia the resistor.

In an electronic circuit according to another aspect of the presentinvention, the electronic circuit may further includes: a first diodehaving an anode connected to the DC power output terminal of the powerconversion circuit and a cathode connected to the power supply terminalof the control circuit; and a second diode having an anode connected toa connection point between the load and the switch and a cathodeconnected to the power supply terminal of the control circuit.

In an electronic circuit according to another aspect of the presentinvention, the power supply may include a positive electrode terminaland a negative electrode terminal, and the power conversion circuit mayinclude a reverse current reducer configured to reduce a current flowingfrom the positive electrode terminal of the power supply to the negativeelectrode terminal of the power supply via the control circuit and thepower conversion circuit.

In an electronic circuit according to another aspect of the presentinvention, the reverse current reducer may supply, to the DC poweroutput terminal, DC electric power, in which electric power has beenconverted, input to the power input terminal and reduces a current fromthe DC power output terminal to the power conversion circuit.

In an electronic circuit according to another aspect of the presentinvention, the reverse current reducer may include a third diode, andthe DC electric power, in which electric power has been converted, inputto the power input terminal may flow through the third diode in aforward direction to be supplied to the DC power output terminal.

In an electronic circuit according to another aspect of the presentinvention, the reverse current reducer may include a transistorconfigured to reduce a current flowing from the DC power output terminalto the power conversion circuit when the control circuit controls aconnection state of the switch to be in a conduction state and tosupply, to the DC power output terminal, the DC electric power, in whichelectric power has been converted, input to the power input terminalwhen the control circuit controls the connection state of the switch tobe in a non-conduction state.

In an electronic circuit according to another aspect of the presentinvention, the transistor may be an n-channel type MOSFET.

In an electronic circuit according to another aspect of the presentinvention, the power conversion circuit may include at least a firstcapacitor which includes a first electrode and a second electrodeconnected to the power input terminal, a fourth diode which has an anodeconnected to a grounding point and a cathode connected to the secondelectrode of the first capacitor, a second capacitor which includes afirst electrode connected to the DC power output terminal and a secondelectrode connected to the grounding point, and a fifth diode which hasan anode connected to the input terminal via a capacitor and a cathodeconnected to a first electrode of the second capacitor, and the reversecurrent reducer may reduce a current flowing from the DC power outputterminal to the grounding point via the fourth diode and the fifthdiode.

In an electronic circuit according to another aspect of the presentinvention, the reverse current reducer may include the fifth diode.

In an electronic circuit according to another aspect of the presentinvention, the reverse current reducer may include the fourth diode.

Also, a module according to another aspect of the present inventionincludes: the above-described electronic circuit; a power supplyconfigured to output DC electric power; and a load driven by DC electricpower supplied from the power supply.

A module according to another aspect of the present invention isaccommodated in a housing having waterproofness.

Furthermore, a system according to another aspect of the presentinvention includes: the above-described module; and a transmitterconfigured to transmit prescribed radio waves to the module.

According to the present invention, it is possible to provide aconvenient electronic circuit in which a switch can be switched usingelectric power obtained by weak radio waves.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an example of a constitution of a latchsystem in an embodiment.

FIG. 2 is a diagram illustrating an example of a constitution of a latchmodule in a first embodiment.

FIG. 3 is a diagram illustrating a first modified example of theconstitution of the latch module in the first embodiment.

FIGS. 4A and 4B are exemplary diagrams illustrating a constitution of acontrol circuit in the first embodiment.

FIG. 5 is a diagram illustrating a second modified example of theconstitution of the latch module in the first embodiment.

FIG. 6 is a diagram illustrating an example of a housing with awaterproof structure in the first embodiment.

FIG. 7 is a diagram illustrating an example of a power conversioncircuit in a second embodiment.

FIG. 8 is a diagram illustrating a first modified example of the powerconversion circuit in the second embodiment.

FIG. 9 is a diagram illustrating a second modified example of the powerconversion circuit in the second embodiment.

FIGS. 10A and 10B are exemplary diagrams illustrating a first antennaand a second antenna in a third embodiment.

FIGS. 11A and 11B are exemplary diagrams illustrating the first antennaand the second antenna in a third embodiment.

FIG. 12 is a diagram illustrating an example of a constitution of alatch module in the third embodiment.

FIG. 13 is a diagram illustrating a first modified example of theconstitution of the latch module in the third embodiment.

FIG. 14 is a diagram illustrating a second modified example of theconstitution of the latch module in the third embodiment.

FIG. 15 is a diagram illustrating a third modified example of theconstitution of the latch module in the third embodiment.

FIGS. 16A and 16B are diagrams illustrating a fourth modified example ofthe constitution of the latch module in the third embodiment.

FIG. 17 is a diagram illustrating a fifth modified example of theconstitution of the latch module in the third embodiment.

FIG. 18 is a diagram illustrating an example of a housing with awaterproof structure in the third embodiment.

FIG. 19 is a diagram illustrating an example of a constitution of alatch system in a fourth embodiment.

FIG. 20 is a diagram illustrating a case in which a first antennareceives radio waves in an example of the constitution of the latchsystem in the fourth embodiment.

FIG. 21 is a diagram illustrating a case in which a second antennareceives radio waves in an example of the constitution of the latchsystem in the fourth embodiment.

FIGS. 22A and 22B are exemplary diagrams illustrating a constitution ofa power detector in the fourth embodiment.

FIGS. 23A and 23B are exemplary diagrams illustrating a constitution ofthe power detector having gain switching in the fourth embodiment.

FIG. 24 is a diagram illustrating an example of a constitution of acircuit of the power detector having gain switching in the fourthembodiment.

FIG. 25 is a diagram illustrating an example of a housing with awaterproof structure in the fourth embodiment.

FIG. 26 is a diagram for explaining an object to be achieved in a fifthembodiment.

FIG. 27 is a diagram illustrating an example of a constitution of alatch module in the fifth embodiment.

FIG. 28 is a diagram illustrating a modified example of a powerconversion circuit in the fifth embodiment.

FIG. 29 is a diagram illustrating a second modified example of a powerconversion circuit in the fifth embodiment.

FIG. 30 is a diagram illustrating a third modified example of a powerconversion circuit in the fifth embodiment.

FIG. 31 is a diagram illustrating a fourth modified example of a powerconversion circuit in the fifth embodiment.

FIG. 32 is a diagram illustrating a fifth modified example of a powerconversion circuit in the fifth embodiment.

FIG. 33 is a diagram illustrating a sixth modified example of a powerconversion circuit in the fifth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[Constitution of Latch System 100]

One constitution of a latch system 100 will be described below withreference to the drawings.

FIG. 1 is a diagram illustrating an example of the constitution of thelatch system 100 in an embodiment. As illustrated in FIG. 1 , the latchsystem 100 includes a transmitter 70 and a latch module 1.

The transmitter 70 is a terminal in which radio waves can betransmitted. Here, for example, the radio waves are radio wavestransmitted by a transmission-side device during wireless communicationperformed in accordance with a communication standard such as Bluetooth(registered trademark) or Wi-Fi (registered trademark). The radio wavesare not limited to a communication standard such as Bluetooth(registered trademark) or Wi-Fi (registered trademark), variouscommunication methods can be adopted for the radio waves, and the radiowaves may be transmitted through communication according to a uniquestandard in which the default communication standard is not satisfied.

For example, the transmitter 70 is a mobile information processingterminal in which wireless communication is possible such asmultifunctional mobile phone terminals (smartphones), mobile phoneterminals, personal digital assistants (PDAs), notebook PCs, and tabletPCs. The transmitter 70 is not limited to a mobile informationprocessing terminal and may be another information processing terminal.

In this example, the transmitter 70 transmits radio waves 71.

The latch module 1 includes a power supply 50, a load 60, and a latchcircuit 10. The latch module 1 receives the radio waves 71 transmittedfrom the transmitter 70.

The power supply 50 is a power supply configured to output directcurrent (DC) electric power. For example, the power supply 50 is abattery such as a lithium battery. When the latch module 1 is asmall-sized device, the power supply 50 may be a battery as installed ina board. The power supply 50 supplies electric power to the load 60.

The load 60 has functions such as communication functions. For example,the load 60 may include a read only memory (ROM) (not shown), a randomaccess memory (RAM) (not shown), and a central processing unit (CPU)(not shown).

The latch circuit 10 is connected between the power supply 50 and theload 60. The power supply 50 supplies electric power to the load 60 viathe latch circuit 10.

The latch circuit 10 receives the radio waves 71 transmitted from thetransmitter 70. The latch circuit 10 receives the radio waves 71transmitted from the transmitter 70 to control a conduction statebetween the power supply 50 and the load 60. Hereinafter, the latchcircuit 10 is also referred to as an “electronic circuit.”

First Embodiment

A first embodiment of the present invention will be described below withreference to the drawings.

FIG. 2 is a diagram illustrating an example of a constitution of thelatch module 1 in the first embodiment. In FIG. 2 , the latch module 1includes a latch circuit 10, a power supply 50, and a load 60.

The latch circuit 10 includes an antenna 140, a power conversion circuit110, a control circuit 120, and a switch 130.

The antenna 140 is connected to the power conversion circuit 110. Theantenna 140 receives radio waves 71 transmitted from a transmitter 70.

The power conversion circuit 110 includes a power input terminal 111 asan input terminal and a DC power output terminal 112 as an outputterminal. Radio waves received by the antenna 140 are input to the powerinput terminal 111. The electric power obtained by the radio wavesreceived by the antenna 140 is converted into DC electric power andoutput. The DC power output terminal 112 outputs the DC electric powerconverted by the power conversion circuit 110. That is to say, the powerconversion circuit 110 includes the power input terminal 111 to whichthe electric power obtained by the radio waves received by the antenna140 capable of receiving radio waves is input and the DC power outputterminal 112 configured to output DC electric power. Furthermore, thepower conversion circuit 110 converts the electric power input to thepower input terminal 111 into DC electric power and the converted DCelectric power is output from the DC power output terminal 112.

The power conversion circuit 110 may include an RF-DC conversion circuit113 and a booster circuit 114.

The RF-DC conversion circuit 113 converts the electric power obtained bythe radio waves input to the power input terminal 111 into DC electricpower. The RF-DC conversion circuit 113 outputs the converted DCelectric power to the booster circuit 114.

The booster circuit 114 steps up a voltage of the DC electric powerconverted by the RF-DC conversion circuit 113. The booster circuit 114outputs the stepped-up electric power via the DC power output terminal112.

The control circuit 120 includes an input terminal 121 connected to theDC power output terminal 112 of the power conversion circuit 110 and anoutput terminal 122 connected to the switch 130 and configured tocontrol a connection state of the switch 130. The control circuit 120includes a power supply terminal 123 as a power supply terminal.

The DC electric power output by the power conversion circuit 110 isinput to the input terminal 121. The output terminal 122 outputs anoutput signal corresponding to a state of the input terminal 121.Electric power is supplied from the power supply 50 to the power supplyterminal 123.

The switch 130 is connected between the power supply 50 configured tooutput DC electric power and the load 60 driven by the DC electric powersupplied from the power supply 50. The switch 130 switches a connectionstate between the power supply 50 and the load 60 from a non-conductionstate to a conduction state.

The non-conduction state is a state in which the supply of electricpower from the power supply 50 to the load 60 is cut off and theconduction state is a state in which electric power is supplied from thepower supply 50 to the load 60.

In this example, the control circuit 120 includes a flip-flop 127. Theflip-flop 127 switches a control signal output from the output terminal122.

Although FIG. 2 illustrates an example in which a D flip-flop (a D-F/F)is utilized for the flip-flop 127, the D-F/F may be composed of anotherflip-flop such as a T type flip-flop.

The control circuit 120 controls a conduction state of the switch 130.To be specific, the control circuit 120 controls the connection state ofthe switch 130 when the power conversion circuit 110 outputs DC electricpower due to the antenna 140 receiving the radio waves 71.

To be more specific, the input terminal 121 of the control circuit 120is connected to a CLK terminal 1271 and a D terminal 1272 of theflip-flop 127. Furthermore, the output terminal 122 of the controlcircuit 120 is connected to a Q terminal 1273 of the flip-flop 127.

Since the input terminal 121 has a low level (the same potential as aground potential) while the antenna 140 is not receiving the radio waves71 (that is, in a state in which the transmitter 70 is distant from thelatch circuit 10 or the radio waves 71 are not being transmitted fromthe transmitter 70), the Q terminal 1273 has the low level held therein.In this state, the switch 130 is operated such that it is brought into anon-conduction state. That is to say, in this state, electric power isnot supplied from the power supply 50 to the load 60.

The RF-DC conversion circuit 113 outputs DC electric power to thebooster circuit 114 while the antenna 140 is receiving the radio waves71 (that is, in a state in which the radio waves 71 are beingtransmitted from the transmitter 70 or the transmitter 70 is broughtcloser to the latch circuit 10). The booster circuit 114 performsstepping-up until the Q terminal 1273 of the flip-flop 127 has achanging threshold value potential or more. At this time, in the powerconversion circuit 110, a potential sufficient to change a state of theQ terminal 1273 is input to the CLK terminal 1271 and the D terminal1272 of the flip-flop 127. Thus, the level of the Q terminal 1273 ischanged to a high level. In this state, the switch 130 is controlled tobe in a conduction state.

If the switch 130 is controlled to be in a conduction state, the powersupply 50 supplies electric power to the load 60 via the switch 130.

Since electric power is supplied from the power supply 50 to theflip-flop 127, even if the antenna 140 transitions to a state in whichthe antenna 140 does not receive the radio waves 71 (that is, a state inwhich the latch circuit 10 does not receive radio waves due to reasonssuch as the transmitter 70 moving to a position in which the transmitteris distance from the latch circuit 10), the Q terminal 1273 continues tooutput a high level.

In this embodiment, when the switch 130 is switched to be in aconduction state, the control circuit 120 keeps the switch 130 in theconductive state using electric power supplied from the power supply 50to the power supply terminal 123.

In this example, since the flip-flop 127 is connected to the powersupply 50 via the power supply terminal 123, a constitution in which theflip-flop 127 with low power consumption is selected to significantlyreduce an influence on a battery lifespan may be provided. For example,a flip-flop with low power consumption of less than 1 μA (microampere)may be selected.

FIG. 3 is a diagram illustrating a first modified example of theconstitution of the latch module 1 in the first embodiment. A latchmodule 1 b illustrated in FIG. 3 is a first modified example of thelatch module 1. A constituent element in the first modified example thatis the same as the latch module 1 described above will be denoted by thesame reference numeral and a description thereof will be omitted. Thelatch module 1 b is different from the latch module 1 described above inthat the latch module 1 b includes a control circuit 120 b instead ofthe control circuit 120.

The control circuit 120 b includes an input terminal 121 b connected toa DC power output terminal 112 of a power conversion circuit 110 and anoutput terminal 122 b connected to a switch 130 and configured tocontrol a connection state of the switch 130. Furthermore, the controlcircuit 120 b includes a power supply terminal 123 b as a power supplyterminal.

DC electric power output by the power conversion circuit 110 is input tothe input terminal 121 b. The output terminal 122 b outputs an outputsignal corresponding to a state of the input terminal 121 b. Electricpower is supplied from a power supply 50 to the power supply terminal123 b.

In this example, the control circuit 120 b includes a power detector126. The power detector 126 includes a detection input terminal 1261, areference input terminal 1262, and a voltage detection output terminal1263 as input/output terminals. The reference input terminal 1262 isconnected to a ground point TG. The voltage detection output terminal1263 outputs a potential corresponding to a potential of the detectioninput terminal 1261 and a potential of the reference input terminal1262.

In a state in which an antenna 140 does not receive radio waves 71 (thatis, in a state in which a transmitter 70 is distance from a latchcircuit 10 or radio waves 71 is not transmitted from the transmitter70), the input terminal 121 b has a low level. Thus, the low level isinput to the detection input terminal 1261. Since the reference inputterminal 1262 is connected to the ground point TG (fixed to the lowlevel), the voltage detection output terminal 1263 outputs the lowlevel. In this state, the switch 130 is controlled to be in anon-conduction state.

When the antenna 140 is receiving radio waves 71 (that is, in a state inwhich radio waves 71 are being transmitted from the transmitter 70 orthe transmitter 70 is brought closer to the latch circuit 10), an RF-DCconversion circuit 113 outputs DC electric power to a booster circuit114. The booster circuit 114 steps up the potential of a power detector126 to an operation potential (a high level) thereof. The powerconversion circuit 110 outputs a high level. Since the high level isinput to the detection input terminal 1261 of the power detector 126,the voltage detection output terminal 1263 outputs a high level. In thisstate, the switch 130 is controlled to be in a conduction state.

In this example, the detection input terminal 1261 of the power detector126 is connected to the voltage detection output terminal 1263.

The control circuit 120 b may include a resistor 124 b. In this case,the detection input terminal 1261 of the power detector 126 is connectedto the voltage detection output terminal 1263 via the resistor 124 b. Asan example, a resistance value of the resistor 124 b may be 10 mega-ohmsor more. When the control circuit 120 b includes the resistor 124 b, itis possible to further reduce power consumption.

When the power detector 126 outputs a high level to control the switch130 to be in a conduction state and thus the power supply 50 isconnected to a load 60, a current flowing through the control circuit120 b is only a current flowing through the resistor 124 b and electriccurrent consumption of the power detector 126, both of which are small.

In this way, since the power consumption of the power detector 126 istheoretically zero, the power consumption of the power supply 50 inwhich the switch 130 is in a non-conduction state can be changed tosubstantially zero. Therefore, in the example in which the powerdetector 126 is provided, it is possible to further reduce powerconsumption as compared with the example described above in which theflip-flop 127 is provided.

FIGS. 4A and 4B are exemplary diagrams illustrating a constitution ofthe power detector 126 in the first embodiment.

FIG. 4A is an example of the constitution of the power detector 126. InFIG. 4A, the power detector 126 includes a transistor Q1, a transistorQ2, a transistor Q3, a transistor Q4, an inverter 1264, and a resistor1265.

The transistor Q1, the transistor Q2, and the transistor Q4 areenhancement type elements. The transistor Q3 is a depression typeelement.

When the output terminal 122 b outputs a low level, the transistor Q2and the transistor Q3 are turned on. When the output terminal 122 boutputs a high level, the transistor Q1 and the transistor Q4 are turnedon.

In this example, a current flows from the power supply terminal 123 b tothe ground point TG through two routes. A first route is a route inwhich a current flows from the power supply terminal 123 b to the groundpoint TG via the transistor Q1 and the transistor Q3 and a second routeis a route in which a current flows from the power supply terminal 123 bto the ground point TG via the transistor Q2 and the transistor Q4. Thepower detector 126 cuts off both of the routes in which a current flowsfrom the power supply terminal 123 to the ground point TG all the time,regardless of the state of the output terminal 122 b.

Therefore, the power consumption of the power detector 126 istheoretically zero.

FIG. 4B is a truth value table in an example of the constitution of thepower detector 126 illustrated in FIG. 4A. FIG. 4B illustrates acorrespondence relationship between potentials of the input terminal 121b and the output terminal 122 b and states of the transistor Q1, thetransistor Q2, the transistor Q3, and the transistor Q4.

IN indicates a level of the potential of the input terminal 121 b andOUT indicates a level of the potential of the output terminal 122 b. Thetransistor Q1, the transistor Q2, the transistor Q3, and the transistorQ4 indicate the states of the transistors.

FIG. 5 is a diagram illustrating a second modified example of theconstitution of the latch module 1 in the first embodiment. A latchmodule 1 c illustrated in FIG. 5 is the second modified example of thelatch module 1. A constituent element in the second modified examplethat is the same as the latch module 1 described above will be denotedby the same reference numeral and a description thereof will be omitted.The latch module 1 c is different from that in a case in which a latchmodule 1 a and a latch module 1 b supply electric power to a powersupply terminal 123 c included in a control circuit 120 c. The controlcircuit 120 c is an example of the control circuit 120.

The control circuit 120 c includes an input terminal 121 c connected toa DC power output terminal 112 of a power conversion circuit 110 and anoutput terminal 122 c connected to a switch 130 and configured tocontrol a connection state of the switch 130. Furthermore, the controlcircuit 120 c includes the power supply terminal 123 c as a power supplyterminal.

DC electric power output by the power conversion circuit 110 is input tothe input terminal 121 c. The output terminal 122 c outputs an outputsignal corresponding to a state of the input terminal 121 c. Electricpower is supplied from at least one of the power conversion circuit 110and the power supply 50 to the power supply terminal 123 c.

In this example, the latch module 1 c includes a diode D1 and a diodeD2. The diode D1 has an anode connected to the DC power output terminal112 of the power conversion circuit 110 and a cathode connected to thepower supply terminal 123 c of the control circuit 120 c. The diode D2has an anode connected to a connection point between the switch 130 andthe load 60 and a cathode connected to the power supply terminal 123 cof the control circuit 120 c. Hereinafter, the diode D1 will be alsoreferred to as a “first diode” and the diode D2 will be also referred toas a “second diode.”

In a state in which the antenna 140 does not receive radio waves 71(that is, in a state in which the transmitter 70 is distance from thelatch circuit 10 or radio waves 71 are not transmitted from thetransmitter 70), electric power is not supplied to the control circuit120 c. In this case, the switch 130 is controlled to be in anon-conduction state. The output terminal 122 c may be fixed at a lowlevel using a resistor (not shown) or the like.

When the antenna 140 is receiving radio waves 71 (that is, in a state inwhich radio waves 71 are being transmitted from the transmitter 70 orthe transmitter 70 is brought closer to the latch circuit 10), an RF-DCconversion circuit 113 outputs DC electric power to the booster circuit114. The booster circuit 114 steps up the DC electric power to apotential of a potential or more obtained by adding an amountcorresponding to a stepped-down voltage of the diode D1 to an operationpotential of the power detector 126. The power conversion circuit 110supplies electric power from the DC power output terminal 112 to thepower supply terminal 123 c of the control circuit 120 c via the diodeD1.

In this case, a potential output from the DC power output terminal 112of the power conversion circuit 110 is also input to the input terminal121 c of the control circuit 120 c. If a high level is input to theinput terminal 121 c, the control circuit 120 c outputs a high level tothe output terminal 122 c. Therefore, the switch 130 is controlled to bein a conduction state.

If the switch 130 is controlled to be in the conduction state, the powersupply 50 supplies electric power to the power supply terminal 123 c ofthe control circuit 120 c via the switch 130 and the diode D2.

Even if the antenna 140 does not receive radio waves 71 (that is, astate in which the latch circuit 10 no longer receives radio waves dueto reasons such as the transmitter 70 moving distance from the latchcircuit 10) transitions, the control circuit 120 c can receive thesupply of electric power from the power supply 50 via the switch 130 andthe diode D2.

Therefore, in this embodiment, if the switch 130 is controlled to be ina conduction state when the antenna 140 receives the radio waves 71, theswitch 130 is kept conductive.

FIG. 6 is a diagram illustrating an example of a housing with awaterproof structure in the first embodiment. As illustrated in FIG. 6 ,a latch waterproof module 2 includes the latch circuit 10, the powersupply 50 configured to output DC electric power, the load 60 configuredto be driven by DC electric power supplied from the power supply 50, anda housing 80.

The housing 80 includes the latch circuit 10, the power supply 50, andthe load 60 accommodated therein. The housing 80 is waterproof.

Summary of Effects of the First Embodiment

According to the embodiment described above, the latch circuit 10controls a connection state between the power supply 50 and the load 60to be in a conduction state using the radio waves received by theantenna 140.

In the related art, in the small-sized device as shipped with thebattery installed in the board, electric power is supplied the momentthe battery is installed, and the consumption of the battery begins.Although it is desirable that electric power be supplied only after thedevice is delivered to a customer or when a customer intends to supplyelectric power in view of a battery lifespan, contact type switches,(removable) insulating films, and the like lead to an increase in sizeof the small-sized device.

The latch circuit 10 can prevent increase in size of the small-sizeddevice by controlling a connection state between the power supply 50 andthe load 60 to be in a conduction state using the radio waves receivedby the antenna 140.

Also, according to the above-described embodiment, in the latch circuit10, the power conversion circuit 110 converts the electric powerobtained by the radio waves received by the antenna 140 into DC electricpower and the control circuit 120 controls a conduction state of theswitch 130. Since the latch circuit 10 can switch a state of the switch130 to be in a conduction state even using weak radio waves whenincluding the control circuit 120, it does not take time to start up theAFE which is a load. Thus, it is possible to provide the latch module 1in which it does not take time from the reception of radio waves to thestarting-up of the load 60.

Therefore, it is possible to provide the convenient latch circuit 10.

Also, according to the above-described embodiment, the control circuit120 included in the latch circuit 10 receives electric power suppliedfrom the power supply 50. Therefore, after the antenna 140 receivesradio waves, the latch circuit 10 can keep the connection state of theswitch 130 conductive even in a state in which the antenna 140 does notreceive radio waves.

Furthermore, according to the above-described embodiment, the controlcircuit 120 includes the flip-flop 127. Therefore, the control circuit120 can switch and keep the connection state of the switch 130 using asimple constitution.

Also, according to the above-described embodiment, the control circuit120 includes the power detector 126 having feedback.

Since the output signal of the control circuit 120 is fed back to theinput, the control circuit 120 can maintain the connection state of theswitch 130. Furthermore, it is possible to maintain the state of theswitch 130 with low power consumption.

Furthermore, according to the above-described embodiment, the powerdetector 126 further includes the resistor 124 b in which 10 mega-ohmsor more is provided. Therefore, when the control circuit 120 includesthe resistor 124 b, it is possible to further reduce electric powerconsumed by the control circuit 120.

In addition, according to the above-described embodiment, when the latchmodule 1 includes the diode D1, the power conversion circuit 110provides electric power of the control circuit 120. Therefore, the latchmodule 1 can obtain electric power for switching the switch 130 to aconduction state from radio waves received by the antenna 140. That isto say, the latch module 1 can switch the switch 130 to a conductionstate without consuming electric power of the power supply 50.

Moreover, according to the above-described embodiment, when the latchmodule 1 includes the diode D2, after the switch 130 is controlled to bein a conduction state, it is possible to receive the supply of electricpower from the power supply 50. Therefore, the switch 130 can maintainthe conduction state.

In addition, according to the above-described embodiment, the latchmodule 1 is accommodated in the housing 80 which is waterproof. Forexample, when the starting-up of the system is required in a non-contactstate in a device used in a sealed state in water, it is possible tostart up the system in a non-contact state by applying the latch module1 in this embodiment.

Examples of the device used in a sealed state in water a water qualitymeasurement device, a small-sized camera device, and the like.Furthermore, the expression “in water” includes not only water but alsoa wide range of liquids such as electrolytic solutions and body fluids.

Second Embodiment

A second embodiment of the present invention will be described belowwith reference to the drawings.

FIG. 7 is a diagram illustrating an example of a power conversioncircuit 110 d in the second embodiment. The power conversion circuit 110d is an example of the power conversion circuit 110.

In FIG. 7 , the power conversion circuit 110 d includes a firstcapacitor C11, a first diode D11, a second diode D12, and a secondcapacitor C12.

The first capacitor C11 includes a first electrode C11 a and a secondterminal C11 b. The first electrode C11 a of the first capacitor C11 isconnected to a power input terminal 111 a and a second electrode C11 bis connected to a connection point between a cathode of the first diodeDl1 and an anode of the second diode D12.

The first diode D11 has an anode connected to a ground point TG and thecathode connected to the second electrode C11 b of the first capacitorC11.

The second capacitor C12 includes a first electrode C12 a and a secondelectrode C12 b. The first electrode C12 a of the second capacitor C12is connected to the DC power output terminal 112 d and the secondelectrode C12 b is connected to the ground point TG.

The second diode D12 has an anode connected to a power input terminal111 d via a capacitor. In the case of this example, the anode of thesecond diode D12 is connected to the power input terminal 111 d via thefirst capacitor C11. Furthermore, the second diode D12 has a cathodeconnected to the first electrode C12 a of the second capacitor C12.

If a positive potential is applied to the power input terminal 111 d, acurrent I11 flows from the power input terminal 111 d via the firstcapacitor C11 and the second diode D12. Electric charges are accumulatedin the second capacitor C12 through the current I11.

If a negative potential is applied to the power input terminal 111 d, acurrent I12 flows from the ground point TG via the first diode D11 andthe first capacitor C11. Electric charges are accumulated in the firstcapacitor C11 through the current I12.

When a positive potential is applied to the power input terminal 111 dagain, a current I11 flows from the power input terminal 111 d via thefirst capacitor C11 and the second diode D12. As a result, a potentialtwice that of the power input terminal 111 d is output to the DC poweroutput terminal 112 d. This is an operation of a half-wave voltagedoubler rectifier circuit.

The power conversion circuit 110 d is a voltage doubler rectifiercircuit configured to step up weak radio waves. The weak radio wavesuses, for example, a 2.4 GHz band such as a Bluetooth (registeredtrademark) low Energy (LE) (hereinafter referred to as a “BLE”) standardcommunication method installed in a smartphone or the like. It isdesirable that the first diode D11 and the second diode D12(hereinafter, when diodes provided in the power conversion circuit 110are not distinguished, they are referred to as a “diode D”) haveexcellent high frequency characteristics, a low forward voltage, and asmall capacitance between terminals. In this example, the diode D may bea Schottky barrier diode.

In the first capacitor C11 and the second capacitor C12 (hereinafter,when capacitors provided in the power conversion circuit 110 are notdistinguished, they are referred to as a “capacitor C”), a response rateand an arrival voltage value of a stepped-up voltage change inaccordance with a capacitance. Furthermore, if the capacitor C does nothave a sufficient capacitance value, an amount of ripple (an amount ofvoltage fluctuation) of an output voltage increases and DCcharacteristics deteriorate. On the other hand, if the capacitor C has atoo large capacitance value, it takes time to charge and responsivenessdeteriorates.

Therefore, the capacitance value of the capacitor C is adjusted to avalue at which well-balanced and good voltage doubler characteristicscan be obtained for each application of the selected diode D and latchcircuit 10. For example, in this embodiment, 33 pF, 24 pF, and the likeare preferable at 2.4 GHz.

An optimum capacitance value changes depending on a stray capacitance, aboard pattern, an installation layout, and the like of a board. In thecircuit illustrated in FIG. 7 , an optimum value changes in the range ofseveral to several tens of pF. An optimum capacitance value needs to beconfirmed in accordance with a stray capacitance, a board pattern, aninstallation layout, and the like of a board.

When the power conversion circuit 110 has a constitution in which theabove-described voltage doubler rectifier circuit is incorporated inmultiple stages, it is possible to obtain a potential at which thecontrol circuit 120 can operate even with weaker radio waves.

FIG. 8 is a diagram illustrating the power conversion circuit 110 ewhich is a first modified example of the power conversion circuit 110 inthe second embodiment. The power conversion circuit 110 e is an exampleof the power conversion circuit 110 described above. A constituentelement in the first modified example that is the same as the powerconversion circuit 110 described above will be denoted by the samereference numeral and a description thereof will be omitted.

In FIG. 8 , the power conversion circuit 110 e includes a firstcapacitor C21, a first diode D21, a second capacitor C22, a second diodeD22, a third capacitor C23, a third diode D23, a fourth capacitor C24,and a fourth diode D24.

In this example, the first capacitor C21, the first diode D21, thesecond capacitor C22, and the second diode D22 are the same as the firstcapacitor C11, the first diode D11, the second capacitor C12, and thesecond diode D12, respectively.

When the power conversion circuit 110 e further includes the thirdcapacitor C23, the third diode D23, the fourth capacitor C24, and thefourth diode D24 as compared with the power conversion circuit 110 d, atwo-stage voltage doubler rectifier circuit is constituted.

The third capacitor C23 includes a first electrode C23 a and a secondelectrode C23 b. The first electrode C23 a of the third capacitor C23 isconnected to a cathode of the third diode D23 and the second electrodeC23 b is connected to a ground point TG.

The third diode D23 has an anode connected to a second electrode C21 bof the first capacitor C21 and the cathode connected to the firstelectrode C23 a of the third capacitor C23.

The fourth capacitor C24 includes a first electrode C24 a and a secondelectrode C24 b. The first electrode C24 a of the fourth capacitor C24is connected to a power input terminal 111 e and the second electrodeC24 b is connected to a cathode of the fourth diode D24.

The fourth diode D24 has an anode connected to a connection pointbetween the cathode of the third diode D23 and the first electrode C23 aof the third capacitor C23 and the cathode connected to the secondelectrode C24 b of the fourth capacitor C24.

Also, in the case of this example, the anode of the second diode D22 isconnected to the power input terminal 111 e via the fourth capacitorC24.

In the example illustrated in FIG. 8 , a voltage four times an inputvoltage input to the power input terminal 111 e can be output from theDC power output terminal 112 e.

When the multi-stage voltage doubler rectifier circuit is constituted inthis way, the power conversion circuit 110 can obtain a potential atwhich the control circuit 120 can operate.

Here, when the multi-stage voltage doubler rectifier circuit isconstituted, it causes a decrease in RF-DC conversion efficiency.Furthermore, the manufacturing cost increases due to an increase in thenumber of parts constituting the multi-stage voltage doubler rectifiercircuit. Therefore, it is desirable to set the number of stagesappropriate for each application.

FIG. 9 is a diagram illustrating a power conversion circuit 110 f whichis a second modified example of the power conversion circuit 110 in thesecond embodiment. The power conversion circuit 110 f is an example ofthe power conversion circuit 110 described above. A constituent elementin the second modified example that is the same as the power conversioncircuit 110 described above will be described by the same referencenumeral and a description thereof will be omitted.

In FIG. 9 , the power conversion circuit 110 f includes a firstcapacitor C31, a first diode D31, a second capacitor C32, a second diodeD32, a third capacitor C33, a third diode D33, a fourth capacitor C34, afourth diode D34, a fifth capacitor C35, a fifth diode D35, a sixthcapacitor C36, and a sixth capacitor C36.

In this example, the first capacitor C31, the first diode D31, thesecond capacitor C32, the second diode D32, the third capacitor C33, thethird diode D33, the fourth capacitor C34, and the fourth diode D34 arethe same as the first capacitor C21, the first diode D21, the secondcapacitor C22, the second diode D22, the third capacitor C23, the firstdiode D23, the fourth capacitor C24, and the fourth diode D24,respectively.

The power conversion circuit 110 f further includes a fifth capacitorC35, a fifth diode D35, a sixth capacitor C36, and a sixth diode D36compared with the power conversion circuit 110 e to constitute athree-stage voltage doubler rectifier circuit.

The fifth capacitor C35 includes a first electrode C35 a and a secondelectrode C35 b. The first electrode C35 a of the fifth capacitor C35 isconnected to a cathode of the fifth diode D35 and the second electrodeC35 b is connected to the ground point TG.

The fifth diode D35 has an anode connected to a second electrode C34 bof the fourth capacitor C34 and the cathode connected to the firstelectrode C35 a of the fifth capacitor C35.

The sixth capacitor C36 includes a first electrode C36 a and a secondelectrode C36 b. The first electrode C36 a of the sixth capacitor C36 isconnected to a power input terminal 111 f and the second electrode C36 bis connected to a cathode of the sixth diode D36.

The sixth diode D36 has an anode connected to a connection point betweenthe cathode of the fifth diode D35 and the first electrode C35 a of thefifth capacitor C35 and the cathode connected to the second electrodeC36 b of the sixth capacitor C36.

Also, in the case of this example, the anode of the second diode D32 isconnected to the power input terminal 111 f via the sixth capacitor C36.

In the example illustrated in FIG. 9 , a voltage which is six times aninput voltage input to the power input terminal 111 f can be output fromthe DC power output terminal 112 f.

When the multi-stage voltage doubler rectifier circuit is constituted inthis way, the power conversion circuit 110 can obtain a potential atwhich the control circuit 120 can operate. In the example associatedwith the power conversion circuit 110 f, the control circuit 120 can beoperated by weaker radio waves as compared with the example associatedwith the power conversion circuit 110 e.

Summary of Effects of Second Embodiment

According to the embodiment described above, the latch circuit 10illustrated in FIG. 1 constitutes the power conversion circuit 110 usingthe voltage doubler rectifier circuit. When the latch circuit 10constitutes the power conversion circuit 110 using the voltage doublerrectifier circuit, it is possible to generate DC electric powerstepped-up from that of weak radio waves. The power conversion circuit110 can drive the control circuit 120 by supplying the stepped-up DCelectric power to the control circuit 120.

Also, according to the above-described embodiment, the voltage doublerrectifier circuit is constituted of the capacitor C and the diode D.Therefore, the latch circuit 10 can constitutes the power conversioncircuit 110 with a simple constitution.

Furthermore, according to the above-described embodiment, the latchcircuit 10 constitutes the power conversion circuit 110 using thetwo-stage voltage doubler rectifier circuit. When the latch circuit 10constitutes the power conversion circuit 110 using the two-stage voltagedoubler rectifier circuit, it is possible to drive the control circuit120 even with weak radio waves. When the latch circuit 10 drives thecontrol circuit 120 even with weak radio waves, it is possible tocontrol the power supply 50 and the load 60 to be in a conduction state.

In addition, according to the above-described embodiment, the latchcircuit 10 constitutes the power conversion circuit 110 using thethree-stage voltage doubler rectifier circuit. When the latch circuit 10constitutes the power conversion circuit 110 using the three-stagevoltage doubler rectifier circuit, it is possible to drive the controlcircuit 120 even with weaker radio waves compared with the two-stagevoltage doubler rectifier circuit. The latch circuit 10 can control thepower supply 50 and the load 60 to be in a conduction state by drivingthe control circuit 120 even with weaker radio waves.

Third Embodiment

A third embodiment of the present invention will be described below withreference to the drawings.

FIGS. 10A and 10B are exemplary diagrams illustrating a first antenna240 and a second antenna 340 in a third embodiment. FIG. 10A is adiagram illustrating an example of a constitution of a case in which thelatch module 1 described above includes one antenna 140. In this case,the radio waves received by the antenna 140 are converted into DCelectric power by the power conversion circuit 110 and input to thecontrol circuit 120.

The control circuit 120 controls the switch 130 from a non-conductionstate to a conduction state. In the case of this example, the controlcircuit 120 cannot control the switch 130 from a conduction state to anon-conduction state.

FIG. 10B is a diagram illustrating an example of a constitution in acase in which the latch module 1 includes two antennas (a first antenna240 and a second antenna 340). In this case, the latch module 1 includesthe first antenna 240, a first power conversion circuit 210, the secondantenna 340, a second power conversion circuit 310, and a controlcircuit 220.

In this example, the first antenna 240 and the second antenna 340 may beprovided at different angles from each other.

The first antenna 240 is provided to be able to receive first radiowaves in a first direction.

The first power conversion circuit 210 includes a first power inputterminal 211 and a first DC power output terminal 212. The first powerinput terminal 211 is connected to the first antenna 240. The first DCpower output terminal 212 is connected to the control circuit 220.

The electric power obtained by the first radio waves received by thefirst antenna 240 is input to the first power input terminal 211. If theelectric power is input to the first power input terminal 211, the firstpower conversion circuit 210 converts the electric power input to thefirst power input terminal 211 into DC electric power. The first powerconversion circuit 210 outputs DC electric power from the first DC poweroutput terminal 212.

The second antenna 340 is provided to be able to receive second radiowaves in a second direction different from the first direction.

The second power conversion circuit 310 includes a second power inputterminal 311 and a second DC power output terminal 312. The second powerinput terminal 311 is connected to the second antenna 340. The second DCpower output terminal 312 is connected to the control circuit 220.

The electric power obtained by the second radio waves received by thesecond antenna 340 is input to the second power input terminal 311. Ifthe electric power is input to the second power input terminal 311, thesecond power conversion circuit 310 converts the electric power input tothe second power input terminal 311 into DC electric power. The secondpower conversion circuit 310 outputs DC electric power from the secondDC power output terminal 312.

The control circuit 220 includes, as input/output terminals, a firstinput terminal 221, a second input terminal 225, and an output terminal222.

The first input terminal 221 is connected to the first DC power outputterminal 212 of the first power conversion circuit 210. The second inputterminal 225 is connected to the second DC power output terminal 312 ofthe second power conversion circuit 310. The output terminal 222 isconnected to the switch 130 and controls the connection state of theswitch 130.

The control circuit 220 controls the connection state of the switch 130to be in a conduction state when the first power conversion circuit 210outputs DC electric power due to the reception of the first radio wavesby the first antenna 240. The control circuit 220 controls theconnection state of the switch 130 to be in a non-conduction state whenthe second power conversion circuit 310 outputs DC electric power due tothe reception of the second radio waves by the second antenna 340.

In this way, in the example illustrated in FIG. 10B, the control circuit220 can not only control the switch 130 from a non-conduction state to aconduction state, but also control the switch 130 from a conductionstate to a non-conduction state.

FIGS. 11A and 11B exemplary diagrams illustrating the first antenna 240and the second antenna 340 in the third embodiment.

FIG. 11A is a diagram illustrating an example of an electric field typeantenna 500 in the third embodiment. FIG. 11A illustrates an example ofa type of antenna when the first antenna 240 or the second antenna 340is the electric field type antenna 500.

When the first antenna 240 or the second antenna 340 is the electricfield type antenna 500, the first antenna 240 or the second antenna 340may be a dipole antenna 501, a monopole antenna 502, an inverted Fantenna 503, a meander line antenna 504, or a chip antenna 505.

FIG. 11B is a diagram illustrating an example of a magnetic field typeantenna 600 in the third embodiment. FIG. 11B illustrates an example ofa type of antenna when the first antenna 240 or the second antenna 340is the magnetic field type antenna 600.

When the first antenna 240 or the second antenna 340 is the magneticfield type antenna 600, the first antenna 240 or the second antenna 340may be a loop antenna 601.

The types of antennas of the first antenna 240 and the second antenna340 in this embodiment are not limited to the types of antennasillustrated in (A) and FIG. 11B. In addition, various antennas can beselected as the first antenna 240 and the second antenna 340.

FIG. 12 is a diagram illustrating an example of a constitution of alatch module 1 g in the third embodiment. The latch module 1 gillustrated in FIG. 6 is a modified example of the latch module 1 in thefirst embodiment. A constituent element in this example that is the sameas the latch module 1 described above will be denoted by the samereference numeral and a description thereof will be omitted.

In FIG. 6 , the latch module 1 g includes a latch circuit 10 g, a powersupply 50, and a load 60.

The latch circuit 10 g includes an electric field type antenna 500 a, amagnetic field type antenna 600 a, a first power conversion circuit 210a, a second power conversion circuit 310 a, a control circuit 220, and aswitch 130. The electric field type antenna 500 a is an example of afirst antenna 240 and the magnetic field type antenna 600 a is anexample of a second antenna 340.

The first power conversion circuit 210 a may include an RF-DC conversioncircuit 213 a and a booster circuit 214 a and the second powerconversion circuit 310 a may include an RF-DC conversion circuit 313 aand a booster circuit 314 a.

The electric field type antenna 500 a receives first radio waves in afirst direction. The electric power when the electric field type antenna500 a receives the first radio waves is input to the first power inputterminal 211 a of the first power conversion circuit 210 a. The firstpower conversion circuit 210 a converts the input electric power into DCelectric power and outputs the converted DC electric power to the firstDC power output terminal 212 a.

In this case, since the magnetic field type antenna 600 a is installedto be able to receive second radio waves in a second direction differentfrom the first direction, the magnetic field type antenna 600 a does notreceive the first radio waves. Therefore, DC electric power is input toonly the first input terminal 221 of the control circuit 220.

The magnetic field type antenna 600 a receives the second radio waves inthe second direction. Electric power when the magnetic field typeantenna 600 a receives the second radio waves is input to the secondpower input terminal 311 a of the second power conversion circuit 310 a.The second power conversion circuit 310 a converts the input electricpower into DC electric power and outputs the converted DC electric powerto the second DC power output terminal 312 a.

In this case, since the electric field type antenna 500 a is installedto be able to receive the first radio waves in the first directiondifferent from the second direction, the electric field type antenna 500a does not receive the second radio waves. Therefore, DC electric poweris input to only the second input terminal 225 of the control circuit220.

In this example, the control circuit 220 includes a flip-flop 227. Theflip-flop 227 switches a control signal outputs from the output terminal222 on the basis of a potential of the first input terminal 221 and apotential of the second input terminal 225.

To be specific, the flip-flop 227 is an SR flip-flop (SR-F/F). To bemore specific, the first input terminal 221 is connected to an Sterminal of the flip-flop 227, the second input terminal 225 isconnected to an R terminal of the flip-flop 227, and the output terminal222 is connected to a Q terminal.

In a state in which the electric field type antenna 500 a receives thefirst radio waves, the first power conversion circuit 210 a outputs DCelectric power. The output DC electric power is input to the S terminalin which a potential according to the DC electric power corresponds tothat of the first input terminal 221 of the flip-flop 227. When theinput potential exceeds a threshold value voltage in which a state ofthe flip-flop 227 is caused to be changed, that is, if a high level isinput to the S terminal of the flip-flop 227, the Q terminal of theflip-flop 227 outputs a high level. In this state, the switch 130 iscontrolled to be in a conduction state. If the switch 130 is controlledto be in the conduction state, the power supply 50 supplies electricpower to the load 60.

In a state in which the magnetic field type antenna 600 a receives thesecond radio waves, the second power conversion circuit 310 a outputs DCelectric power. The output DC electric power is input to the R terminalin which a potential according to the DC electric power corresponds tothat of the second input terminal 225 of the flip-flop 227. When theinput potential exceeds a threshold value voltage in which a state ofthe flip-flop 227 is caused to be changed, that is, if a high level isinput to the R terminal of the flip-flop 227, the Q terminal of theflip-flop 227 outputs a low level. In this state, the switch 130 iscontrolled to be in a non-conduction state.

Electric power is supplied from the power supply 50 to the controlcircuit 220 including the flip-flop 227. Therefore, in a state in whichboth of the electric field type antenna 500 a and the magnetic fieldtype antenna 600 a do not receive radio waves, the flip-flop 227continues to keep an output state of the Q terminal. That is to say, aconnection state of the switch 130 differs depending on whether it iscontrolled to be in a conduction state or in a non-conduction state inaccordance with whether the electric field type antenna 500 a or themagnetic field type antenna 600 a finally receives radio waves. If theswitch 130 is controlled to be in the non-conduction state, the powersupply 50 stops the supply of electric power to the load 60.

In this example, since the flip-flop 227 is connected to the powersupply 50 via a power supply terminal 223, a constitution in which theflip-flop 227 with low power consumption is selected to significantlyreduce an influence on a battery lifespan may be provided. For example,a flip-flop with low power consumption of less than 1 μA (microampere)may be selected.

Also, the control circuit 220 may be composed of a low power consumptionlatch circuit having a function equivalent to that of a flip-flop.

FIG. 13 is a diagram illustrating a first modified example of theconstitution of the latch module 1 in the third embodiment. A latchmodule 1 h illustrated in FIG. 7 is a modified example of the latchmodule 1 g described above. A constituent element in this example thatis the same as the latch module 1 g will be denoted by the samereference symbol and a description thereof will be omitted. The latchmodule 1 h has a constitution different from that of the latch module 1g in that a power detector 226 is included in a control circuit 220 anda flip-flop 227 is not included.

A constitution of the control circuit 220 included in the latch module 1h is the same as that of the control circuit 120 b illustrated in FIG. 3. That is to say, a constitution of the power detector 226 included inthe control circuit 220 is the same as that of the power detector 126illustrated in FIGS. 4A and 4B.

In the example associated with the latch module 1 h, a first DC poweroutput terminal 212 a of a first power conversion circuit 210 a isconnected to a first input terminal 221 of the control circuit 220. Thatis to say, when an electric field type antenna 500 a receives firstradio waves, a current proportional to electric power due to thereceived radio waves is input to a detection input terminal 2261 of thepower detector 226. In this case, when a potential proportional to thecurrent input to an input terminal 2261 is higher than a reference inputpotential VDET in the power detector 226, a high level is output to avoltage detection output terminal 2263 and a switch 130 is controlled tobe in a conduction state.

Also, a second DC power output terminal 312 a of a second powerconversion circuit 310 a is connected to a second input terminal 225 ofthe control circuit 220. That is to say, when a magnetic field typeantenna 600 a receives second radio waves, a current proportional to theelectric power obtained by the received radio waves is input to areference input terminal 2262 of the power detector 226. In the powerdetector 226, a current amplifier is present in the next stage of aninput terminal 2262. A current input to the reference input terminal2262 is amplified to be doubled using the current amplifier.Furthermore, a current adder in the next stage subtracts a current inputto the reference input terminal 2262 from a current input to thedetection input terminal 2261. When a potential proportional to thecurrent which has passed through the current adder is lower than thereference input potential VDET in the power detector 226, a low level isoutput to the voltage detection output terminal 2263 and the switch 130is controlled to be in a non-conduction state.

In this way, the latch module 1 h including the control circuit 220having the power detector 226 operates in the same manner as in thelatch module 1 g including the control circuit 220 a having theflip-flop 227.

When the latch module 1 h includes the power detector 226 in the controlcircuit 220, it is possible to further reduce power consumption ascompared with the latch module 1 g.

FIG. 14 is a diagram illustrating a second modified example of theconstitution of the latch module 1 in a third embodiment. A latch module1 i illustrated in FIG. 14 is a modified example of the latch module 1 gdescribed above. A constituent element in the second modified examplethat is the same as the latch module 1 g will be denoted by the samereference symbol and a description thereof will be omitted. The latchmodule 1 i has a different constitution from the latch module 1 g inthat an electric field type antenna 500 is used for both a first antenna240 and a second antenna 340.

In the example illustrated in FIG. 14 , the electric field type antenna500 b (the first antenna 240) and the electric field type antenna 500 c(the second antenna 340) are provided at different positions from eachother. That is to say, in the latch module 1 i, installation positionsof the antennas are different from each other. Each of the installationpositions is a position in the latch module 1 i in which the antenna isinstalled.

The electric field type antenna 500 b is installed to be able to receivefirst radio waves in a first direction. The electric field type antenna500 c is installed to be able to receive second radio waves in a seconddirection different from the first direction.

FIG. 15 is a diagram illustrating a second modified example of theconstitution of the latch module 1 in the third embodiment. A latchmodule 1 j illustrated in FIG. 15 is a third modified example of thelatch module 1 i described above. A constituent element in this examplethat is the same as the latch module 1 i will be denoted by the samereference symbol and a description thereof will be omitted. The latchmodule 1 j has a constitution different from that of the latch module 1i in that installation angles of the first antenna 240 and the secondantenna 340 are different from each other.

In this example illustrated in FIG. 15 , the latch module 1 j includesan electric field type antenna 500 d and an electric field type antenna500 e. The electric field type antenna 500 d is an example of the firstantenna 240 and the electric field type antenna 500 e is an example ofthe second antenna 340.

In the example illustrated in FIG. 15 , the electric field type antenna500 b (the first antenna 240) and the electric field type antenna 500 c(the second antenna 340) are provided at different angles. That is tosay, since the first radio waves and the second radio waves are providedat different angles, the first radio waves and the second radio waves donot interfere with each other.

As an example, the electric field type antenna 500 b (the first antenna240) and the electric field type antenna 500 c (the second antenna 340)may be provided perpendicular to each other.

FIGS. 16A and 16B are diagrams illustrating a fourth modified example ofthe constitution of the latch module 1 in the third embodiment.

FIG. 16A is a diagram illustrating the arrangement of an antenna 701 andan antenna 702 using a two-dimensional Cartesian coordinate system of anx axis and a y axis. The antenna 701 illustrated in FIG. 9 is an exampleof the first antenna 240 described above and the antenna 702 is anexample of the second antenna 340 described above. Hereinafter, when theantenna 701 and the antenna 702 are not distinguished, the antenna 701and the antenna 702 are referred to as an “antenna 700.” The antenna 700is an example of the electric field type antenna 500.

The antenna 701 is arranged along the x axis. The antenna 702 isarranged along the y axis. In this example, the antenna 701 and theantenna 702 are arranged at different angles from each other.

FIG. 16B is a diagram illustrating the arrangement of the antenna 701and the antenna 702 using a three-dimensional Cartesian coordinatesystem of an x axis, a y axis, and a z axis. In FIG. 16B, thearrangement of the antenna 701 and the antenna 702 illustrated using thetwo-dimensional Cartesian coordinate system of the x axis and the y axisin FIG. 16A is illustrated in a three-dimensional space.

The antenna 701 and the antenna 702 are accommodated in a housing 703.Furthermore, the antenna 701 and the antenna 702 are arranged in thesame plane. When the antenna 701 and the antenna 702 are arranged atdifferent angles from each other as illustrated in FIG. 16B, even whenthe antenna 701 and the antenna 702 are arranged on the same plane, thefirst radio waves and the second radio waves do not interfere with eachother.

FIG. 17 is a diagram illustrating a fifth modified example of theconstitution of the latch module 1 in the third embodiment. A latchmodule 1 k illustrated in FIGS. 10A and 10B is a modified example of thelatch module 1 j described above. A constituent element in this examplethat is the same as the latch module 1 j will be denoted by the samereference symbols and a description thereof will be omitted. The latchmodule 1 k has a constitution different from that of the latch module 1j in that, although the installation angles of the first antenna 240 andthe second antenna 340 are the same, the first antenna 240 and thesecond antenna 340 include the dipole antennas 501 having differentlengths from each other.

In the example illustrated in FIG. 17 , the latch module 1 k includes anelectric field type antenna 500 f and an electric field type antenna 500g. The electric field type antenna 500 f is an example of the firstantenna 240 and the electric field type antenna 500 g is an example ofthe second antenna 340.

In this example, the electric field type antenna 500 f and the electricfield type antenna 500 g includes antennas having lengths different fromeach other. In order to prevent the interference between the first radiowaves and the second radio waves, the length of the electric field typeantenna 500 f and the length of the electric field type antenna 500 gare selected on the basis of frequency of the radio waves. For example,the length of the electric field type antenna 500 f and the length ofthe electric field type antenna 500 g are preferably ½ or ¼ ofwavelengths λ of the radio waves received by the antennas. In this case,the electric field type antenna 500 f and the electric field typeantenna 500 g can efficiently receive radio waves without generatingreflected waves.

To be specific, when the frequency of the first radio waves is 2.4 GHzand the frequency of the second radio waves is 5 GHz, a wavelength ofthe first radio waves is about 12.5 cm and a wavelength of the secondradio waves is about 6 cm. Furthermore, when a dipole antenna with awavelength of λ/2 is utilized, in each antenna, the length of theantenna configured to receive the first radio waves is 6.25 cm and thelength of the antenna configured to receive the second radio waves is 3cm.

As described above, when a constitution in which a latch module 1 k inwhich the dipole antenna 501 having an antenna length of ½ of thewavelengths λ of the first radio waves and the second radio waves ofdifferent wavelengths is utilized is provided, it is possible to preventthe interference between the first radio waves and the second radiowaves. In this case, one of the radio waves can be used for turning onthe switch 130 and the other of the radio waves can be used for turnedoff the switch 130.

Although a case in which a length of the first antenna 240 and thelength of the second antenna 340 are different has been described usingan example of the dipole antenna 501, the same applies not only to anexample of the dipole antenna 501, but also to the monopole antenna 502,the inverted F antenna 503, the meander line antenna 504, and the chipantenna 505. Similarly, these antennas may be configured so that thelength of the antenna included in the first antenna 240 and the lengthof the antenna included in the second antenna 340 are different.

FIG. 18 is a diagram illustrating an example of a housing having awaterproof structure in the third embodiment. As illustrated in FIGS.11A and 11B, a latch waterproof module 2 b includes a latch circuit 101,a power supply 50 configured to output DC electric power, a load 60driven by DC electric power supplied from the power supply 50, and ahousing 80.

The housing 80 includes the latch circuit 101, the power supply 50, andthe load 60 accommodated therein. The housing 80 is waterproof.

Summary of Effects of Third Embodiment

According to the embodiment described above, the latch module 1 switchesa connection state of the switch 130 by detecting the first radio wavesreceived by the first antenna 240 and the second radio waves received bythe second antenna 340. The latch module 1 can switch the switch 130from a non-conduction state to a conduction state when configured inthis way. Furthermore, the latch module 1 can switch the switch 130 froma conduction state to a non-conduction state.

Also, according to the above-described embodiment, the first antenna 240and the second antenna 340 are provided at different positions from eachother. Therefore, the latch module 1 can prevent the first radio wavesin the first direction and the second radio waves in the directiondifferent from the first direction from interfering with each other.That is to say, it is possible to prevent malfunction.

Furthermore, according to the above-described embodiment, the firstantenna 240 and the second antenna 340 are provided at different anglesfrom each other. Therefore, the latch module 1 can prevent the firstradio waves in the first direction and the second radio waves in thedirection different from the first direction from interfering with eachother. That is to say, it is possible to prevent malfunction.

In addition, according to the above-described embodiment, the firstantenna 240 and the second antenna 340 are provided perpendicular toeach other. Therefore, according to the above-described embodiment, itis possible to prevent the first radio waves and the second radio wavesfrom interfering with each other. That is to say, it is possible toprevent malfunction.

Moreover, according to the above-described embodiment, the first antenna240 is the electric field type antenna 500 and the second antenna 340 isthe magnetic field type antenna 600. Therefore, it is possible toprevent the first radio waves and the second radio waves frominterfering with each other. That is to say, it is possible to preventmalfunction.

Fourth Embodiment

A fourth embodiment of the present invention will be described belowwith reference to the drawings.

FIG. 19 is a diagram illustrating an example of a constitution of alatch system 100 in the fourth embodiment. In FIG. 19 , a latch module 1m includes a first antenna 740 a, a second antenna 740 b, a first powerconversion circuit 710 a, a second power conversion circuit 710 b, acontrol circuit 720, a switch 130, a power supply 50, and a load 60. Thefirst power conversion circuit 710 a may include an RF-DC conversioncircuit 713 a and a booster circuit 714 a and the second powerconversion circuit 710 b may include an RF-DC conversion circuit 713 band a booster circuit 714 b.

The first antenna 740 a is provided to be able to receive radio waves.The second antenna 740 b is provided to be able to receive radio wavesand has the same characteristics and gain as in the first antenna 740 a.The first antenna 740 a and the second antenna 740 b are arrangeddistance from each other at a predetermined distance.

In this example, a transmission point T1 indicates a point far from thepoint at which the first antenna 740 a is provided and the point atwhich the second antenna 740 b is provided. To be specific, the antennasare installed in places in which the radio waves transmitted from thetransmission point T1 is in a far field.

Radio waves are divided into a far field and a near field in accordancewith a distance from the transmission point. For example, a boundaryposition between a far field and a near field is represented by λ/2πusing a wavelength λ of radio waves. As an example, at 2.4 GHz, about 2cm from a transmission point is a boundary between a far field and anear field.

Since radio waves can be regarded as plane waves in a far field, ifdistances from a transmission point are the same, an electric fieldintensity and a magnetic field intensity are theoretically the same. Anintensity is inversely proportional to the first power of distance.

On the other hand, in a near field, in an electric field antenna, anelectric field intensity is inversely proportional to the third power ofdistance and a magnetic field intensity is inversely proportional to thesecond power of distance. In addition, in a magnetic field antenna, anelectric field intensity is inversely proportional to the second powerof distance and a magnetic field intensity is inversely proportional tothe third power of distance. That is to say, an electric field intensityand a magnetic field intensity in a near field have a significantlylarger change in intensity depending on a distance than in a far field.

When radio waves from a far field is received, the radio waves seen fromthe first antenna 740 a and the second antenna 740 b are plane waves andmagnitudes of electric powers P1 and P2 received by the antennas aresubstantially the same.

In this embodiment, the first antenna 740 a and the second antenna 740 bhave substantially the same characteristics and gain. Thus, when each ofthe first antenna 740 a and the second antenna 740 b receives radiowaves from the transmission point T1, the DC electric power output bythe first power conversion circuit 710 a and the DC electric poweroutput by the second power conversion circuit 710 b are substantiallythe same.

In this example, radio waves from a far field are regarded as noise. Theradio waves from the far field are radio waves floating in theenvironment. For example, when the latch module 1 m receives the radiowaves from the far field, a malfunction in which a connection state ofthe switch 130 is switched is likely to be caused.

The first power conversion circuit 710 a includes a first power inputterminal 711 a to which electric power obtained when the first antenna740 a receives radio waves input and a first DC power output terminal712 a configured to output DC electric power, converts electric powerinput to the first power input terminal 711 a into DC electric power,and outputs the converted DC electric power from the first DC poweroutput terminal 712 a.

The second power conversion circuit 710 b includes a second power inputterminal 711 b to which electric power obtained when the second antenna740 b receives radio waves is input and a second DC power outputterminal 712 b configured to output DC electric power, converts electricpower input to the second power input terminal 711 b into DC electricpower, and outputs the converted DC electric power from the second DCpower output terminal 712 b.

The control circuit 720 includes a first input terminal 721 connected tothe first DC power output terminal 712 a of the first power conversioncircuit 710 a, a second input terminal 725 connected to the second DCpower output terminal 712 b of the second power conversion circuit 710b, an output terminal 722 connected to the switch 130 and configured tocontrol a connection state of the switch 130, and a power supplyterminal 723.

The power supply terminal 723 of the control circuit 720 is connected tothe power supply 50. The load 60 is connected to the power supply 50 viathe switch 130.

The control circuit 720 switches a connection state of the switch 130 inaccordance with the result of comparing the electric power input to thefirst input terminal 721 with the electric power input to the secondinput terminal 725.

FIG. 20 is a diagram illustrating a case in which the first antenna 740a receives radio waves having an intensity higher than that of thesecond antenna 740 b in an example of the constitution of the latchsystem 100 in the fourth embodiment. A constituent element in this casethat is the same as the latch module 1 m described above will be denotedby the same reference symbol and a description thereof will be omitted.

In this example, a transmission point T2 is located in a near field ofthe first antenna 740 a. In the near field, a change in electric fieldintensity and magnetic field intensity depending on a distance is large.Thus, the electric power due to the radio waves received by the firstantenna 740 a is significantly different from the electric power due tothe radio waves received by the second antenna 740 b.

For example, if radio waves are transmitted in the vicinity of the firstantenna 740 a, an electric field intensity and a magnetic fieldintensity in the vicinity of the second antenna 740 b are significantlyattenuated with respect to an electric field intensity and a magneticfield intensity in the vicinity of the first antenna 740 a.

When the transmission point T2 is located in the near field of the firstantenna 740 a, the electric power due to the radio waves received by thefirst antenna 740 a is significantly different from the electric powerdue to the radio waves received by the second antenna 740 b. Thus, theDC electric power output by the first power conversion circuit 710 a issignificantly different from the DC electric power output by the secondpower conversion circuit 710 b. That is to say, the electric power inputto the first input terminal 721 of the control circuit 720 issignificantly different from the electric power input to the secondinput terminal 725.

The control circuit 720 switches a connection state of the switch 130 inaccordance with the result of comparing the electric powers input to thefirst input terminal 721 and the second input terminal 725. Thus, whenthe transmission point T2 is located in the near field of the firstantenna 740 a, a connection state of the switch 130 is switched.

For example, the control circuit 720 controls the switch 130 to be in aconduction state when the electric power input to the first inputterminal 721 is twice or more the electric power input to the secondinput terminal 725.

FIG. 21 is a diagram illustrating a case in which the second antenna 740b receives radio waves having an intensity higher than that of the firstantenna 740 a in an example of the constitution of the latch system 100in the fourth embodiment. A constituent element in this case that is thesame as the latch module 1 m described above will be denoted by the samereference symbol and a description thereof will be omitted.

In this example, a transmission point T3 is located in a near field ofthe second antenna 740 b. If radio waves are transmitted from thetransmission point T3 near the second antenna 740 b, an electric fieldintensity and a magnetic field intensity in the vicinity of the firstantenna 740 a are significantly attenuated with respect to an electricfield intensity and a magnetic field intensity in the vicinity of thesecond antenna 740 b.

For example, when the electric power input to the second input terminal725 is larger than the electric power input to the first input terminal721 by a predetermined amount or more, the control circuit 720 controlsthe switch 130 to be in a non-conduction state.

As illustrated in FIGS. 20 and 21 , when radio waves in a near field aretransmitted near either the first antenna 740 a or the second antenna740 b, it is possible for the latch circuit 10 to detect a position of atransmitter and it is possible for the latch circuit 10 to switch aconnection state of the switch 130 without a malfunction.

FIGS. 22A and 22B are exemplary diagrams illustrating a constitution ofthe control circuit 720 in the fourth embodiment.

FIG. 22A is a diagram illustrating an example of a circuit constitutionof the control circuit 720 in the fourth embodiment. As illustrated inFIG. 22A, the control circuit 720 is constituted of a power detector 726and a feedback resistor 724. The power detector 726 includes a firstinput terminal 7212, a second input terminal 7252, and the outputterminal 7222 as input/output terminals. The power detector 726 has theoutput terminal 7222 connected to the first input terminal 7212 via thefeedback resistor 724. The feedback resistor 724 feeds back the electricpower of the output terminal 7222 of the power detector 726 to an inputterminal 7212. For this reason, if the output terminal 7222 is a highlevel, the power detector 726 maintains a high level of an outputterminal 7212 unless a large amount of electric power is input to thesecond input terminal 7252 to make the switch 130 non-conductive. Thepower detector 726 includes a current amplifier 7261, a current adder7262, and a current comparator 7263 as constituent elements. The powerdetector 726 compares the electric powers input to the first inputterminal 7212 and the second input terminal 7252 by comparing thecurrents input as values proportional to the electric powers input tothe first input terminal 7212 and the second input terminal 7252.

The current amplifier 7261 amplifies a current I_(INM) input to thesecond input terminal 7252. In this example, the current amplifier 7261amplifies the current input to the second input terminal 7252 G times.

The current adder 7262 adds a current (G×I_(INM)) obtained by amplifyinga current (G×I_(INM)) input to the second input terminal 7252 G timesusing the current amplifier 7261 and a current INP input to the firstinput terminal 7212. The current adder 7262 outputs the added current.

The current comparator 7263 compares the current (I_(INP)−G×I_(INM))output as a result of addition by the current adder 7262 with thedetected current I_(DET). The current comparator 7263 outputs a voltagecorresponding to the comparison result to the output terminal 7222.

To be specific, the current comparator 7263 outputs a high level whenthe current output as a result of addition by the current adder 7262 isthe detected current I_(DET) or more and outputs a low level when thecurrent output as a result of addition by the current adder 7262 is lessthan the detected current I_(DET). Hereinafter, the current comparator7263 be also referred to as a “comparator.”

That is to say, the current comparator 7263 outputs a high level when adifference between a current flowing through the first input terminal7212 and a current obtained by amplifying a current flowing through thesecond input terminal 7252 G times using the current amplifier 7261 isthe detected current I_(DET) or more. Furthermore, the currentcomparator 7263 outputs a low level when a difference between a currentflowing through the first input terminal 7212 and a current obtained byamplifying a current flowing through the second input terminal 7252 Gtimes using the current amplifier 7261 is less than the detected currentI_(DET).

For example, when a gain (an amplification factor) of the currentamplifier 7261 is set to a gain (an amplification factor) which is twicethe gain (the amplification factor), the current comparator 7263 doesnot output a high level unless the current flowing through the firstinput terminal 7212 is a value or more obtained by adding the detectedcurrent I_(DET) to a current obtained by doubling the current flowingthrough the second input terminal 7252.

Hereinafter, a state in which the current comparator 7263 outputs a lowlevel to the output terminal 7222 is also referred to as an “off state”and a state in which the current comparator 7263 outputs a high level tothe output terminal 7222 is also referred to as an “on state.”

FIG. 22B is a table showing a correspondence relationship between acurrent input to the power detector 726 and an output potential.

When a current value obtained by subtracting a current obtained byamplifying the current flowing through the second input terminal 7252 Gtimes from the current flowing through the first input terminal 7212 isthe detected current I_(DET) or more, the output terminal 7222 outputs ahigh level. Since the output terminal 7222 is connected to the switch130, in this case, the control circuit 720 controls the switch 130 to bein a conduction state.

When a current value obtained by subtracting the current obtained byamplifying the current flowing through the second input terminal 7252 Gtimes from the current flowing through the first input terminal 7212 isless than the detected current I_(DET), the output terminal 7222 outputsa low level. Since the output terminal 7222 is connected to the switch130, in this case, the control circuit 720 controls the switch 130 to bein a non-conduction state.

FIGS. 23A and 23B are exemplary diagrams illustrating a constitution ofthe power detector 726 a having gain (amplification factor) switching inthe fourth embodiment. The power detector 726 a is an example of thepower detector 726. A constituent element in this example that is thesame as the power detector 726 will be denoted by the same referencenumeral and a description thereof will be omitted.

An example of a case in which the power detector 726 a switches a gainwill be described with reference to FIGS. 23A and 23B. As an example, acase in which, when the power detector 726 a is in an off state, a gainis doubled to prevent a malfunction, whereas when the power detector 726a is in an on state, a gain is set to a gain which is ½ times the gainso that it is difficult to shift to an off state will be described.

In this example, the power detector 726 a includes a gain switch 7264.

When the second input terminal 7252 of the power detector 726 a isconnected to the current amplifier 7261, the current flowing through thesecond input terminal 7252 can obtain the doubled gain.

When the power detector 726 a is in an off state and receives radiowaves transmitted from a far field, the currents input to the firstinput terminal 7212 and the second input terminal 7252 are substantiallythe same. Thus, a current of the second input terminal 7252 having thedoubled gain becomes larger and a potential of the output terminal 7222is maintain in an off state. That is to say, when the latch module 1 mincludes the power detector 726 a, it is possible to prevent amalfunction.

On the other hand, when the power detector 726 a receives radio waves ina near field to be in an on state, the power detector 726 a can bemaintained in an on state by performing setting so that the current INPflowing through the first input terminal 7212 is larger than the currentI_(INP) flowing through the second input terminal 7252.

For example, if the gain (amplification factor) connected to the secondinput terminal 7252 is changed from 2 times to ½, even if the currentinput to the first input terminal 7212 and the current input to thesecond input terminal 7252 are substantially the same, the currentflowing through the first input terminal 7212 becomes larger. Thus, itbecomes difficult for the power detector 726 a to shift to an off state.That is to say, the latch module 1 m can be easily maintained in the onstate.

FIG. 24 is a diagram illustrating an example of the circuit constitutionof the power detector 726 a having the gain switching in the fourthembodiment. A constituent element in this example that is the same asthe power detector 126 described in the first embodiment will be denotedby the same reference numeral and a description thereof will be omitted.In FIG. 24 , the power detector 726 a further includes a currentamplifier 7261, a current adder 7262, and a gain switch 7264.

The current amplifier 7261 includes a transistor Q5 and a transistor Q6.Both of the transistor Q5 and the transistor Q6 are n-channel typetransistors. The transistor Q5 has a source connected to a ground pointTG, a gate connected to a drain of the transistor Q5 and a gate of thetransistor Q6, and the drain connected to the second input terminal7252. The transistor Q6 has a source connected to the ground point TG,the gate connected to the gate of the transistor Q5, and a drainconnected to the current adder 7262. The transistor Q5 and thetransistor Q6 constitute a current mirror circuit. The current I_(INM)input to the second input terminal 7252 flows between the drain and thesource of the transistor Q5 as a current I1. In this example, ½×I1 flowsbetween the drain and the source of the transistor Q6 as a current I2.

The gain switch 7264 includes a transistor Q7 and a transistor Q8. Thetransistor Q7 is an n-channel type transistor and the transistor Q8 is ap-channel type transistor.

The transistor Q7 has a source connected to the ground point TG, a gateconnected to the gate of the transistor Q6, and a drain connected to adrain of the transistor Q8.

The transistor Q8 has a source connected to a connection point betweenthe drain of the transistor Q6 and the current adder 7262, a gateconnected to the output terminal 7222, and the drain connected to thedrain of the transistor Q7. In this example, 3/2×I1 flows between thedrain and the source of the transistor Q7 as a current I3.

The gain switch 7264 switches a current value of the current flowingthrough the current adder 7262 by controlling the current I3 inaccordance with the state of the output terminal 7222.

When the output terminal 7222 is in a low level, the current I3 flowsbetween the source and the drain of the transistor Q8. In this case,since I2+I3 (that is, ½×I1+3/2×I1=2×I1) flows through the current adder7262, the gain is doubled.

When the output terminal 7222 is in a high level, a current does notflow between the source and the drain of the transistor Q8. In thiscase, since I2 (that is, ½×I1) flows through the current adder 7262, thegain is halved.

Here, a current flowing between a drain and a source of a MOS transistoris proportional to a gate width W and inversely proportional to a gatelength L.

A value of the gain of the power detector 726 a is arbitrarily adjustedusing the gate width W and the gate length L of the MOS transistorconstituting the transistor Q6 and the transistor Q7.

FIG. 25 is a diagram illustrating an example of a housing 80 with awaterproof structure in the fourth embodiment. As illustrated in FIG. 25, a latch waterproof module 2 c includes a latch circuit 10 p, a powersupply 50 configured to output DC electric power, a load 60 driven by DCelectric power supplied from the power supply 50, and the housing 80.

The housing 80 includes the latch circuit 10 p, the power supply 50, andthe load 60 accommodated therein. The housing 80 is waterproof.

Summary of Effects of Fourth Embodiment

According to the embodiment described above, the latch module 1 mincludes the first antenna 740 a and the second antenna 740 b havingsubstantially the same characteristics and gain as in the first antenna740 a. The control circuit 720 compares the electric power due to theradio waves received by the first antenna 740 a with the electric powerdue to the radio waves received by the second antenna 740 b.

When a difference between the electric power due to the radio wavesreceived by the first antenna 740 a and the electric power due to theradio waves received by the second antenna 740 b is less than apredetermined value, the transmission point of radio waves isconceivable to be in a far field. Thus, the control circuit 720 does notswitch the connection state of the switch 130. When a difference betweenthe electric power due to the radio waves received by the first antenna740 a and the electric power due to the radio waves received by thesecond antenna 740 b is a predetermined value or more, the transmissionpoint of radio waves is conceivable to be in a near field. Thus, thecontrol circuit 720 switches the connection state of the switch 130.

Therefore, the latch module 1 m can detect that the noise radio wavesfrom the far field is not near the transmitter regardless of theintensity of radio waves and can prevent a malfunction due to the radiowaves from the far field.

Also, since these controls utilize the electric field characteristics ofthe transmission radio waves, power consumption of the battery of thecircuit does not occur for detection.

Furthermore, according to the above-described embodiment, the controlcircuit 720 switches the connection state of the switch 130 to aconduction state when the electric power input from the first inputterminal 721 is larger than the electric power input from the secondinput terminal 725 and switches the connection state of the switch 130to a non-conduction state when the electric power input from the firstinput terminal 721 is smaller than the electric power input from thesecond input terminal 725.

Therefore, when the latch module 1 m includes the control circuit 720,it is possible to switch the power supply 50 and the load 60 between aconduction state and a non-conduction state.

Furthermore, according to the above-described embodiment, the controlcircuit 720 includes the power detector 726. Therefore, the controlcircuit 720 including the power detector 726 can maintain the state ofthe switch 130 with low power consumption.

In addition, according to the above-described embodiment, when the powerdetector 726 includes the current amplifier 7261, in a case in whichthere is a difference between the electric power input to the firstinput terminal 7212 and the electric power input to the second inputterminal 7252, the connection state of the switch 130 is switched.

The connection state of the switch 130 is switched only when thedifference of a predetermined value or more set using the gain of thecurrent amplifier 7261 is detected. Thus, the power detector 726 canprevent a malfunction.

Moreover, according to the above-described embodiment, the powerdetector 726 a can switch the gain when including the gain switch 7264.It is possible to switch the gain between when the power detector 726 ais in the on state and when the power detector 726 a is in the offstate.

In a state in which the power detector 726 a is in the off state, whenthe power detector 726 a increases a weight of the gain, the powerdetector 726 a can be easily maintained in the off state and can preventa malfunction.

In a case in which the power detector 726 a is in the on state, when thepower detector 726 a reduces a weight of the gain, the power detector726 a can be easily maintained in the on state.

According to the above-described embodiment, the first antenna 740 a andthe second antenna 740 b are arranged apart from each other at apredetermined distance. The latch module 1 m switches the connectionstate of the switch 130 when the transmitter 70 is brought closer to thefirst antenna 740 a or the second antenna 740 b.

Therefore, the position in which the transmitter 70 is brought closer tothe antenna to make the connection state of the switch 130 conductive isdifferent from the position in which the transmitter 70 is broughtcloser to the antenna to make the connection state of the switch 130non-conductive. Thus, it is possible to prevent a malfunction.

Fifth Embodiment

A fifth embodiment of the present invention will be described below withreference to the drawings.

First, an object to be achieved in the fifth embodiment will bedescribed. FIG. 26 is a diagram for explaining the object to be achievedin the fifth embodiment. In FIG. 26 , the object to be achieved in thefifth embodiment will be described with reference to the constitution ofthe latch module 1 b described with reference to FIG. 3 .

A control circuit 120 b includes an input terminal 121 b, an outputterminal 122 b, and a power supply terminal 123 b, as input/outputterminals. Electric power is supplied from a power supply 50 to thepower supply terminal 123 b and the control circuit 120 b controls theoutput terminal 122 b in accordance with the state of the input terminal121 b. In this case, a current I51 flows from a positive electrodeterminal of the power supply 50 to the control circuit 120 b.

The control circuit 120 b includes an electric power detector 126including a detection input terminal 1261, a reference input terminal1262, and a voltage detection output terminal 1263, as input/outputterminals. The electric power detector 126 is controlled using electricpower supplied to the power supply terminal 123 b. The electric powerdetector 126 outputs a potential according to a potential of thedetection input terminal 1261 and a potential of the reference inputterminal 1262 to the voltage detection output terminal 1263. In thiscase, a current I52 flows from the voltage detection output terminal1263. The current I52 branches into a current I53 flowing through thedetection input terminal 1261 and a current I54 flowing through a powerconversion circuit 110. Here, since the current I54 flowing through thepower conversion circuit 110 is an unnecessary current, an object ofthis embodiment is to reduce the current I54. In the followingdescription, the current I54 may be referred to as a “reverse current”in some cases.

FIG. 27 is a diagram illustrating an example of a constitution of alatch module 1 p in the fifth embodiment. The latch module 1 p is amodified example of the latch module 1 b described with reference toFIG. 3 . The latch module 1 p and the latch module 1 b differ in that apower conversion circuit 860 is provided instead of the power conversioncircuit 110. Constituent elements in the description of the latch module1 p that are the same as those of the latch module 1 b will be denotedby the same reference symbols and description thereof will be omitted.

The power conversion circuit 860 includes an power input terminal 861 asan input terminal and a DC power output terminal 862 as an outputterminal. The radio waves received by an antenna 140 are input to thepower input terminal 861. The power conversion circuit 860 convertselectric power obtained by the radio waves received by the antenna 140into the DC current and outputs the converted electric power. The DCpower output terminal 862 outputs the DC electric power converted by thepower conversion circuit 860.

The power conversion circuit 860 includes a capacitor (first capacitor)C41, a diode (fourth diode) D41, a capacitor (second capacitor) C42, adiode (fifth diode) D42, and a reverse current reducer 863.

The capacitor C41 includes a first electrode C41 a and a second terminalC41 b. The first electrode C41 a of the capacitor C41 is connected tothe power input terminal 861 and the second electrode C41 b is connectedto a connection point between a cathode of the diode D41 and an anode ofthe diode D42. The diode D41 has an anode connected to a grounding pointTG and the cathode connected to the second electrode C41 b of thecapacitor C41. The capacitor C42 includes a first electrode C42 a and asecond electrode C42 b. The first electrode C42 a of the capacitor C42is connected to a first terminal 8631 of the reverse current reducer 863and the second electrode C42 b is connected to the grounding point TG.The anode of the diode D42 is connected to the power input terminal 861via the capacitor C41. Furthermore, a cathode of the diode D42 isconnected to the first electrode C42 a of the capacitor C42.

The reverse current reducer 863 includes an input terminal 8631 and anoutput terminal 8632. The reverse current reducer 863 allows a currentto flow from the input terminal 8631 to the output terminal 8632, butdoes not allow a current to flow from the output terminal 8632 to theinput terminal 8631. That is to say, the reverse current reducer 863supplies, to the DC power output terminal 862, the DC electric power, inwhich electric power has been converted, input to the power inputterminal 861 and reduces a current from the DC power output terminal 862to the power conversion circuit 860. Therefore, the reverse currentreducer 863 can reduce a current from the positive electrode terminal ofthe power supply 50 to a negative electrode terminal of the power supply50 via the control circuit 120 b and the power conversion circuit 860.

As an example, the reverse current reducer 863 includes a diode (thirddiode) D43. The DC electric power, in which electric power has beenconverted, input to the power input terminal 861 flows through the diodeD43 in a forward direction to be supplied to the DC power outputterminal 862. The diode D43 may be, for example, a Schottky barrierdiode for frequencies having a small reverse current.

FIG. 28 is a diagram illustrating a first modified example of the powerconversion circuit in the fifth embodiment. A power conversion circuit870 will be described with reference to FIG. 28 . The power conversioncircuit 870 is a modified example of the power conversion circuit 860.Constituent elements in the description of a latch module 1 q that arethe same as those of the latch module 1 p will be denoted by the samereference symbols and description thereof will be omitted in some cases.The latch module 1 q and the latch module 1 p differ in that the powerconversion circuit 870 is provided instead of the power conversioncircuit 860.

The power conversion circuit 870 includes a capacitor C51, a diode D51,a capacitor C52, and a diode D52 instead of the capacitor C41, the diodeD41, the capacitor C42, and the diode D42 in the power conversioncircuit 860, respectively.

In this embodiment, the diode D52 has a function as a reverse currentreducer 873. The diode D52 may be, for example, a Schottky barrier diodefor high frequencies having a small reverse current.

FIG. 29 is a diagram illustrating a second modified example of the powerconversion circuit in the fifth embodiment. A power conversion circuit880 will be described with reference to FIG. 29 . The power conversioncircuit 880 is a modified example of the power conversion circuit 860.Constituent elements in the description of a latch module 1 r that arethe same as those of the latch module 1 p will be denoted by the samereference symbols and description thereof will be omitted in some cases.The latch module 1 r and the latch module 1 p differ in that the powerconversion circuit 880 is provided instead of the power conversioncircuit 860.

The power conversion circuit 880 includes a capacitor C61, a diode D61,a capacitor C62, and a diode D62 instead of the capacitor C41, the diodeD41, the capacitor C42, and the diode D42 in the power conversioncircuit 870, respectively.

In this embodiment, the diode D61 has a function as the reverse currentreducer 873. The diode D61 may be, for example, a Schottky barrier diodefor high frequencies having a small reverse current.

FIG. 30 is a diagram illustrating a third modified example of the powerconversion circuit in the fifth embodiment. A power conversion circuit890 will be described with reference to FIG. 30 . The power conversioncircuit 890 is a modified example of the power conversion circuit 860.Constituent elements in the description of a latch module is that arethe same as those of the latch module 1 p will be denoted by the samereference symbols and description thereof will be omitted in some cases.The latch module is and the latch module 1 p differ in that the powerconversion circuit 890 is provided instead of the power conversioncircuit 860.

The power conversion circuit 890 includes a capacitor C71, a diode D71,a capacitor C72, and a diode D72 instead of the capacitor C41, the diodeD41, the capacitor C42, and the diode D42 in the power conversioncircuit 870, respectively.

In this embodiment, a reverse current reducer 893 includes a transistorQ11. The transistor Q11 may be, for example, an n-channel typemetal-oxide-semiconductor field-effect transistor (MOSFET). When thetransistor Q11 is an n-channel type MOSFET, the transistor Q11 functionsas a diode due to the short-circuiting between a gate and a sourcethereof. The source of the transistor Q11 is connected to an inputterminal 8931 of the reverse current reducer 893 and a drain of thetransistor Q11 is connected to an output terminal 8932 of the reversecurrent reducer 893.

FIG. 31 is a diagram illustrating a fourth modified example of the powerconversion circuit in the fifth embodiment. A power conversion circuit890A will be described with reference to FIG. 31 . The power conversioncircuit 890A is a modified example of the power conversion circuit 890.Constituent elements in the description of a latch module it that arethe same as those of the latch module 1 s will be denoted by the samereference symbols and description thereof will be omitted in some cases.The latch module 1 t and the latch module is differ in that the powerconversion circuit 890A is provided instead of the power conversioncircuit 890.

The power conversion circuit 890A and the power conversion circuit 890differ in that a reverse current reducer 893A is provided instead of thereverse current reducer 893. Constituent elements in the description ofthe power conversion circuit 890A that are the same as those of thepower conversion circuit 890 will be denoted by the same referencesymbols and description thereof will be omitted in some cases.

The reverse current reducer 893A includes a transistor Q12. Thetransistor Q12 may be, for example, a p-channel type MOSFET. When thetransistor Q12 is a p-channel type MOSFET, the transistor Q12 functionsas a diode due to the short-circuiting between a gate and a sourcethereof. The source of the transistor Q12 is connected to an outputterminal 8932A of the reverse current reducer 893A and a drain of thetransistor Q12 is connected to an input terminal 8931A of the reversecurrent reducer 893A.

FIG. 32 is a diagram illustrating a fifth modified example of the powerconversion circuit in the fifth embodiment. A power conversion circuit890B will be described with reference to FIG. 32 . The power conversioncircuit 890B is a modified example of the power conversion circuit 890.Constituent elements in the description of a latch module 1 u that arethe same as those of the latch module 1 s will be denoted by the samereference symbols and description thereof will be omitted in some cases.The latch module 1 u and the latch module is differ in that the powerconversion circuit 890B is provided instead of the power conversioncircuit 890.

The power conversion circuit 890B and the power conversion circuit 890differ in that a reverse current reducer 893B is provided instead of thereverse current reducer 893. Constituent elements in the description ofthe power conversion circuit 890B that are the same as those of thepower conversion circuit 890 will be denoted by the same referencesymbols and description thereof will be omitted in some cases.

The reverse current reducer 893B includes a transistor Q13 and aninverter INV. The transistor Q13 may be, for example, an n-channel typeMOSFET. In this embodiment, a conduction state of the transistor Q13 iscontrolled by controlling a gate of the transistor Q13 in accordancewith a state of a voltage detection output terminal 1263 included in theelectric power detector 126. When the transistor Q13 is an n-channeltype MOSFET, the gate of the transistor Q13 is connected to the voltagedetection output terminal 1263 via the inverter INV.

When the control circuit 120 b controls a switch 130 such that it isbrought into a conduction state (when the voltage detection outputterminal 1263 outputs a high level), the inverter INV outputs a lowlevel, the transistor Q13 is controlled such that it is brought into anon-conduction state, and a current does not flow through the transistorQ13. Since a current does not flow through the transistor Q13, a reversecurrent flowing to a grounding point TG via a resistor 124 b, a diodeD72, and a diode D71 is reduced. At that time, since a voltage input toan input terminal 1261 of the control circuit 120 b is maintained at ahigh level using the resistor 124 b, a conduction state of the switch130 is maintained independently of the supply of electric power from aDC power output terminal 892A.

When the control circuit 120 b controls the switch 130 such that it isbrought into a non-conduction state (when the voltage detection outputterminal 1263 outputs a low level), the inverter INV outputs a highlevel, the transistor Q13 is controlled to be in a conduction state, anda current flows through the transistor Q13. Since the voltage detectionoutput terminal 1263 is in a low level, a current does not flow to thegrounding point TG via the resistor 124 b, the diode D72, and the diodeD71. At that time, the transistor Q13 is controlled to be in aconduction state, which can perform a preparation for the next on statewithout interfering with a current between the power conversion circuit890B and the control circuit 120 b.

That is to say, the reverse current reducer 893B reduces a currentflowing from a DC power output terminal 892B to the power conversioncircuit 890B when the control circuit 120 b controls a connection stateof the switch 130 to be in a conduction state and supplies, to the DCpower output terminal 892B, the DC electric power, in which electricpower has been converted, input to an power input terminal 891B when thecontrol circuit 120 b controls the connection state of the switch 130 tobe in a non-conduction state.

FIG. 33 is a diagram illustrating a sixth modified example of the powerconversion circuit in the fifth embodiment. A power conversion circuit890C will be described with reference to FIG. 33 . The power conversioncircuit 890C is a modified example of the power conversion circuit 890B.Constituent element in the description of a latch module 1 v that arethe same as those of the latch module 1 u will be denoted by the samereference symbols and description thereof will be omitted in some cases.The latch module 1 v and the latch module 1 u differ in that the powerconversion circuit 890C is provided instead of the power conversioncircuit 890B.

The power conversion circuit 890C and the power conversion circuit 890Bdiffer in that a reverse current reducer 893C is provided instead of thereverse current reducer 893B. Constituent elements in the description ofthe power conversion circuit 890C that are the same as those of thepower conversion circuit 890B will be denoted by the same referencesymbols and description thereof will be omitted in some cases.

The reverse current reducer 893C includes a transistor Q14. Thetransistor Q14 may be, for example, a p-channel type MOSFET. In thisembodiment, a conduction state of the transistor Q14 is controlled bycontrolling a gate of the transistor Q14 in accordance with a state of avoltage detection output terminal 1263 included in an electric powerdetector 126. When the transistor Q14 is a p-channel type MOSFET, thegate of the transistor Q14 is connected to the voltage detection outputterminal 1263.

When a control circuit 120 b controls a switch 130 to be in a conductionstate (when the voltage detection output terminal 1263 outputs a highlevel), the transistor Q14 is controlled to be in a non-conduction stateand a current does not flow through the transistor Q14. Since a currentdoes not flow through the transistor Q14, a reverse current flowing to agrounding point TG via a resistor 124 b, a diode D72, and a diode D71 isreduced. At that time, since a voltage input to an input terminal 1261of the control circuit 120 b is maintained at a high level by theresistor 124 b, a conduction state of the switch 130 is maintainedwithout depending on the supply of electric power from a DC power outputterminal 892C.

When the control circuit 120 b controls the switch 130 to be in anon-conduction state (when the voltage detection output terminal 1263outputs a low level), the transistor Q14 is controlled to be in aconduction state and a current flows through the transistor Q14. Sincethe voltage detection output terminal 1263 is in a low level, a currentdoes not flow to the grounding point TG via the resistor 124 b, thediode D72, and the diode D71. At that time, since the transistor Q14 iscontrolled to be in a conduction state, the transistor Q13 is controlledto be in a conduction state, which can perform a preparation for thenext on state without interfering with a current between the powerconversion circuit 890C and the control circuit 120 b.

That is to say, the reverse current reducer 893C reduces a currentflowing from the DC power output terminal 892C to the power conversioncircuit 890C when the control circuit 120 b controls a connection stateof the switch 130 to be in a conduction state and supplies, to the DCpower output terminal 892C, the DC electric power, in which electricpower has been converted, input to an power input terminal 891C when thecontrol circuit 120 b controls the connection state of the switch 130 tobe in a non-conduction state.

Since an inverter INV can be omitted as compared with the constitutionof the power conversion circuit 890B using the constitution of the powerconversion circuit 890C, it is possible to form a circuit using a smallnumber of constituent elements.

Summary of Effects of Fifth Embodiment

According to the above-described embodiment, in the latch module 1 p,the power conversion circuit 860 includes the reverse current reducer863. The reverse current reducer 863 reduces a current flowing from thepositive electrode terminal of the power supply 50 to the negativeelectrode terminal of the power supply 50 via the control circuit 120 band the power conversion circuit 860. Therefore, according to thisembodiment, it is possible to reduce electric power consumption. Forexample, when the reverse current reducer 863 is not provided, a currentflowing from the positive electrode terminal of the power supply 50 tothe negative electrode terminal of the power supply 50 via the controlcircuit 120 b and the power conversion circuit 860 is about 100 nA(nanoamperes). On the other hand, when the reverse current reducer 863is provided, it is possible to reduce the current to about 8 nA.

Also, according to this embodiment, since the electric power consumptionof the power supply 50 can be reduced, it is possible to increase abattery lifespan of the power supply 50.

Furthermore, according to the above-described embodiment, the reversecurrent reducer 863 supplies, to the DC power output terminal 862, theDC electric power, in which electric power has been converted, input tothe power input terminal 861. In addition, the reverse current reducer863 reduces a current flowing from the DC power output terminal 862 tothe power conversion circuit 860. Therefore, the reverse current reducer863 can reduce a reverse current without affecting a rectifier circuit.

Here, it is desirable that, as a diode used for a rectifier circuit, aSchottky barrier diode which has a low voltage drop and a quick recoverybe utilized so that detection is possible even using little electricpower. According to this embodiment, since the Schottky barrier does notaffect the rectifier circuit, it is possible to perform detection evenwhen a small amount of electric power is received by the antenna 140.

Also, according to the above-described embodiment, the reverse currentreducer 863 may be a Schottky barrier diode for high frequencies. It isdesirable that the Schottky barrier diode for high frequencies having asmall reverse current be provided. Therefore, when the Schottky barrierdiode for high frequencies is adopted as the reverse current reducer863, it is possible to reduce a reverse current.

According to the above-described embodiment, in the latch module 1 q,the power conversion circuit 870 includes the reverse current reducer873. The reverse current reducer 873 reduces a current flowing from a DCpower output terminal 872 to a grounding point via the diode (fourthdiode) D51. Therefore, according to this embodiment, it is possible toreduce a reverse current and reduce electric power consumption.

According to the above-described embodiment, in the latch module 1 q,the power conversion circuit 870 can reduce a reverse current withoutadding a new element as the reverse current reducer 873. Therefore,according to this embodiment, it is possible to reduce a size of acircuit and reduce the manufacturing costs.

According to the above-described embodiment, in the latch module 1 q,the reverse current reducer 873 includes the diode (fifth diode) D52.Thus, according to this embodiment, the power conversion circuit 870 canreduce a reverse current without adding a new element as the reversecurrent reducer 873. Therefore, according to this embodiment, it ispossible to reduce a size of a circuit and reduce the manufacturingcosts.

According to the above-described embodiment, in the latch module ir, areverse current reducer 883 includes the diode (fourth diode) D61. Thus,according to this embodiment, the power conversion circuit 870 canreduce a reverse current without adding a new element as the reversecurrent reducer 883. Therefore, according to this embodiment, it ispossible to reduce the size of a circuit and reduce manufacturing costs.

While the embodiments of the present invention have been described indetail above with reference to the drawings, the specific constitutionis not limited to the embodiments and designs and the like within arange that does not depart from the gist of the present invention arealso included. Furthermore, while the operation has been described froma current according to the electric power using the radio waves receivedby the antennas in the embodiments of the present invention, theoperation may be described from a voltage according to the electricpower using the radio waves received by the antennas.

While preferred embodiments of the invention have been described andillustrated above, it should be understood that these are exemplary ofthe invention and are not to be considered as limiting. Additions,omissions, substitutions, and other modifications can be made withoutdeparting from the spirit or scope of the present invention.Accordingly, the invention is not to be considered as being limited bythe foregoing description, and is only limited by the scope of theappended claims.

What is claimed is:
 1. An electronic circuit, comprising: a switch whichis connected between a power supply configured to output direct current(DC) electric power and a load driven by DC electric power supplied fromthe power supply and which switches a connection state between the powersupply and the load from a non-conduction state in which a supply ofelectric power from the power supply to the load is cut off to aconduction state in which the supply of electric power from the powersupply to the load is allowed; a power conversion circuit which includesa power input terminal to which electric power obtained by radio wavesreceived by an antenna capable of receiving radio waves is input and aDC power output terminal configured to output DC electric power andwhich converts electric power input to the power input terminal into DCelectric power and outputs the converted DC electric power from the DCpower output terminal; and a control circuit which includes an inputterminal connected to the DC power output terminal of the powerconversion circuit and an output terminal connected to the switch andconfigured to control the connection state of the switch and controlsthe connection state of the switch such that it is brought into theconduction state when the power conversion circuit outputs DC electricpower due to reception of radio waves by the antenna, wherein: thecontrol circuit comprises an electric power detector which comprises adetection input terminal and a voltage detection output terminalconfigured to output a potential according to a potential of thedetection input terminal above a threshold, wherein the electric powerdetector comprises a reference input terminal, and the voltage detectionoutput terminal is configured to output the potential according to thepotential of the detection input terminal and a potential of thereference input terminal, the control circuit further comprises aresistor which connects the detection input terminal and the voltagedetection output terminal, and when the switch is switched to be in theconduction state, the control circuit keeps the switch in the conductivestate by the resistor feeding back to the detection input terminal afterthe reception of radio waves ceases.
 2. The electronic circuit accordingto claim 1, wherein: the detection input terminal is connected to thevoltage detection output terminal.
 3. The electronic circuit accordingto claim 2, wherein: a resistance value of the resistor is 10 mega-ohmsor more.
 4. The electronic circuit according to claim 1, furthercomprising: a first diode having an anode connected to the DC poweroutput terminal of the power conversion circuit and a cathode connectedto a power supply terminal of the control circuit; and a second diodehaving an anode connected to a connection point between the load and theswitch and a cathode connected to the power supply terminal of thecontrol circuit.
 5. The electronic circuit according to claim 1, whereinthe power supply includes a positive electrode terminal and a negativeelectrode terminal, and the power conversion circuit includes a reversecurrent reducer configured to reduce a current flowing from the positiveelectrode terminal of the power supply to the negative electrodeterminal of the power supply via the control circuit and the powerconversion circuit.
 6. The electronic circuit according to claim 5,wherein the reverse current reducer supplies, to the DC power outputterminal, DC electric power, in which electric power has been converted,input to the power input terminal and reduces a current from the DCpower output terminal to the power conversion circuit.
 7. The electroniccircuit according to claim 6, wherein the reverse current reducerincludes a third diode, and the DC electric power, in which electricpower has been converted, input to the power input terminal flowsthrough the third diode in a forward direction to be supplied to the DCpower output terminal.
 8. The electronic circuit according to claim 6,wherein the reverse current reducer includes a transistor configured toreduce a current flowing from the DC power output terminal to the powerconversion circuit when the control circuit controls the connectionstate of the switch to be in the conduction state, and configured tosupply, to the DC power output terminal, the DC electric power, in whichelectric power has been converted, input to the power input terminalwhen the control circuit controls the connection state of the switch tobe in the non-conduction state.
 9. The electronic circuit according toclaim 8, wherein the transistor is an n-channel type MOSFET.
 10. Theelectronic circuit according to claim 5, wherein the power conversioncircuit includes at least a first capacitor which includes a firstelectrode connected to the power input terminal and a second electrode,a fourth diode which has an anode connected to a grounding point and acathode connected to the second electrode of the first capacitor, asecond capacitor which includes a first electrode connected to the DCpower output terminal and a second electrode connected to the groundingpoint, and a fifth diode which has an anode connected to the power inputterminal via the first capacitor and a cathode connected to the firstelectrode of the second capacitor, and the reverse current reducerreduces a current flowing from the DC power output terminal to thegrounding point via the fourth diode and the fifth diode.
 11. Theelectronic circuit according to claim 10, wherein the reverse currentreducer includes the fifth diode.
 12. The electronic circuit accordingto claim 10, wherein the reverse current reducer includes the fourthdiode.
 13. A module, comprising: the electronic circuit according toclaim 1; the power supply configured to output DC electric power; andthe load driven by DC electric power supplied from the power supply. 14.The module according to claim 13, wherein the module is accommodated ina waterproof housing.
 15. A system, comprising: the module according toclaim 13; and a transmitter configured to transmit prescribed radiowaves to the module.
 16. A system, comprising: the module according toclaim 14; and a transmitter configured to transmit prescribed radiowaves to the module.